How to Build an NFT Marketplace Backend with RPC Nodes

Every NFT marketplace, whether it's a niche collection hub or a full-scale trading platform, depends on the same underlying data pipeline: read ownership from the blockchain, fetch metadata, track transfers in real time, and execute trades.

The quality of that pipeline determines whether your platform shows stale data or live data, whether listings update in seconds or minutes, and whether users trust your platform enough to trade on it.

This guide walks through the architecture, code patterns, and scaling strategies behind NFT platform backends, covering everything from ownership verification to multi-chain collection indexing.

The Core Data Loop: What Your Platform Needs from the Blockchain

An NFT marketplace backend is essentially a loop that reads, indexes, and serves blockchain data. Understanding the specific RPC methods behind each operation helps you plan your infrastructure correctly from the start.

Ownership verification

It is the foundation. Every time someone views an NFT listing, your platform needs to confirm who currently owns it. This happens through eth_call against the ERC-721 ownerOf function (or balanceOf for ERC-1155 tokens). These calls need to return current data; showing a stale owner after a sale has already settled is a fast way to lose user trust.

Metadata retrieval

It is a two-step process:

You call the contract's tokenURI function via eth_call to get the URI pointing to the token's metadata.
You fetch the actual metadata JSON from wherever it's hosted, usually IPFS or an HTTP server. The on-chain call is fast; the IPFS fetch is often the bottleneck.

Transfer tracking

It is what keeps your platform's database in sync with on-chain reality. You use eth_getLogs to scan for Transfer events, either historically (to index an entire collection from mint to present) or in real time via WebSocket subscriptions. This is also the most RPC-intensive operation you'll perform, especially when indexing large collections with thousands of tokens.

Trade execution

It is the actual buying, selling, and bidding. It uses eth_sendRawTransaction to submit signed transactions to the network. Latency matters here: a slow RPC endpoint means your users' bids arrive late.

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Ownership and Metadata: The Read Path

The read path is where most of your RPC calls will go. Here's a practical implementation using ethers.js that handles ownership checks and metadata fetching:

import { Contract, JsonRpcProvider } from "ethers";
const provider = new JsonRpcProvider("https://go.getblock.io/<YOUR_ACCESS_TOKEN>/");

const ERC721_ABI = [
  "function ownerOf(uint256 tokenId) view returns (address)",
  "function tokenURL(uint256 tokenId) view returns (string)",
  "function balanceOf(address owner) view returns (uint256)",
  "function totalSupply() view returns (uint256)",
];

const nftContract = new Contract(NFT_ADDRESS, ERC721_ABI, provider);

// Verify current ownership
const owner = await nftContract.ownerOf(tokenId);
console.log(`Token #${tokenId} owned by: ${owner}`);

// Fetch metadata: on-chain URI first, then off-chain JSON
const tokenURI = await nftContract.tokenURI(tokenId);
const metadata = await fetch(
 tokenURI.replace("ipfs://", "https://ipfs.io/ipfs/")
)

This works perfectly for individual lookups. For example, a user viewing a single NFT page.

But marketplaces need to display entire collections, and fetching metadata one token at a time doesn't scale. Here's a batched approach that parallelizes the work:

async function fetchCollectionMetadata(contract, tokenIds) {
const BATCH_SIZE = 50;
const results = [];
  for (let i = 0; i < tokenIds.length; i += BATCH_SIZE) {
  const batch = tokenIds.slice(i, i + BATCH_SIZE);
    const uris = await Promise.all(
      batch.map((id) => contract.tokenURI(id).catch(() => null))
);
    const metadatas = await Promise.all(
uris.map(async (uri) => {
        if (!uri) return null;
        try {
     const resp = await fetch(resolveURI(uri));
          return await resp.json();
        } catch {
          return null;
  }
}) 
);
     results.push(...metadatas); 
}
    return results;
}

Batching 50 tokenURI calls at a time strikes a balance between throughput and not overwhelming your RPC endpoint. For a 10,000-item collection, this means 200 batches, which at reasonable concurrency completes in under a minute rather than the 10+ minutes sequential fetching would take.

Collection Indexing: The Historical Scan

Before your marketplace can display a collection, you need to index it, including every mint, every transfer, and every current owner. This means scanning historical Transfer events from the contract's deployment block to the present.

This is where access to archived data becomes essential. Standard full nodes only store recent state; you need archive access to query events from months or years ago. GetBlock provides access to archive nodes only for the paid plans(Shared and Dedicated Node)

Here's a Python implementation that indexes all mints for a collection by scanning for Transfer events from the zero address:

from web3 import Web3
w3 = Web3(Web3.HTTPProvider("https://go.getblock.io/<YOUR_ACCESS_TOKEN>/"))
def index_collection(contract_address, from_block, to_block):
    # Index all mints for a collection by scanning Transfer events from the zero address.
 transfer_topic = w3.keccak(
        text="Transfer(address,address,uint256)"
    ).hex()
 zero_address = "0x" + "0" * 64  # from = 0x0 means mint
    all_mints = []
    chunk_size = 2000  # Stay within getLogs block range limits
  for start in range(from_block, to_block, chunk_size):
        end = min(start + chunk_size - 1, to_block)
        logs = w3.eth.get_logs({
            "address": contract_address,
            "topics": [transfer_topic, zero_address],
            "fromBlock": start,
            "toBlock": end,
        })
for log in logs:
            token_id = int(log["topics"][3].hex(), 16)
            to_address = "0x" + log["topics"][2].hex()[-40:]
            all_mints.append({
                "token_id": token_id,
                "minted_to": to_address,
                "block": log["blockNumber"],
                "tx_hash": log["transactionHash"].hex(),
            })
    return all_mints

The chunk_size of 2,000 blocks per request is important. Most RPC providers (including GetBlock) limit the block range for eth_getLogs queries to prevent a single request from consuming excessive resources. Chunking your scans into manageable ranges keeps your requests within those limits and avoids timeouts on large collections.

Once you've indexed the initial mint history, you need to track all subsequent transfers to maintain an accurate ownership map. You can run the same scanning logic with the zero_address topic filter removed to capture all transfers, not just mints.

Real-Time Transfer Monitoring: Staying in Sync

After your initial index is built, you need to keep it up to date. WebSocket subscriptions let you receive transfer events as they happen, rather than polling for changes on a schedule:

const { WebSocketProvider, ethers } import "ethers";
const wsProvider = new WebSocketProvider(
 "wss://go.getblock.io/<YOUR_ACCESS_TOKEN>/"
);
const transferFilter = {
  address: NFT_CONTRACT_ADDRESS,
  topics: [ethers.id("Transfer(address,address,uint256)")],
};
wsProvider.on(transferFilter, (log) => {
  const from = "0x" + log.topics[1].slice(-40);
  const to = "0x" + log.topics[2].slice(-40);
  const tokenId = BigInt(log.topics[3]);
  console.log(`NFT #${tokenId}: ${from} → ${to}`);
  // Update ownership in your database
  updateOwnership(NFT_CONTRACT_ADDRESS, tokenId, to);
  // Notify users watching this token or collection
  notifyWatchers(NFT_CONTRACT_ADDRESS, tokenId, from, to);
});

This is the pattern that powers real-time listing updates. When someone buys an NFT on-chain, your WebSocket listener picks up the Transfer event within seconds, updates the database, and pushes the change to your frontend.

Without this, you'd need to poll on a timer, and a 30-second polling interval means your marketplace can show "available" listings that were already bought half a minute ago.

Multi-Chain NFT Support

NFTs aren't limited to Ethereum anymore. Collections exist on Polygon, Base, Arbitrum, BSC, Solana, Avalanche, Optimism, and other chains. A competitive marketplace needs to support at least the major EVM chains, and ideally Solana as well for its Metaplex-standard NFTs.

The architectural pattern is straightforward: each chain gets its own RPC endpoint and service module, and a router layer directs queries to the right chain based on the collection being viewed.

const CHAIN_RPCS = {
  ethereum: "https://go.getblock.io/<ETH_TOKEN>/",
  polygon:  "https://go.getblock.io/<POL_TOKEN>/",
  base:     "https://go.getblock.io/<BASE_TOKEN>/",
  arbitrum: "https://go.getblock.io/<ARB_TOKEN>/",
};
async function checkNFTOwnershipAcrossChains(walletAddress) {
  const results = {};
  for (const [chain, rpc] of Object.entries(CHAIN_RPCS)) {
    const provider = new JsonRpcProvider(rpc);
    results[chain] = await queryNFTsForWallet(
      provider, walletAddress, COLLECTIONS[chain]
    );
  }
  return results;
}

GetBlock supports over 130 chains, including all the major NFT ecosystems, from a single dashboard. This means you manage a single set of API keys, a single billing account, and a single support relationship, regardless of how many chains your marketplace covers. As NFT activity shifts between chains (and it does, frequently), you can add new chain support without onboarding a new infrastructure provider.

Conclusion

Building an NFT marketplace backend comes down to mastering four operations: verifying ownership, fetching metadata, indexing collections, and tracking transfers in real time. The code patterns are well-established, and the RPC methods are standardized across EVM chains. The differentiator is infrastructure, whether your platform can handle the burst of a trending mint, the sustained load of indexing a 100,000-token collection, and the real-time responsiveness that makes users trust your listings.

GetBlock provides the multi-chain coverage, archive data access, and WebSocket support that NFT platforms need, starting from a free tier and scaling to dedicated infrastructure as your marketplace grows.

Ready to start building? Get your free API key and connect your NFT platform to the blockchain.