Web3 & DApp Development Fundamentals · Lesson

Marketplaces

Listing and transfers.

Marketplaces is a free Web3 & DApp Development Fundamentals lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web3 & DApp Development Fundamentals learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

How NFT Marketplaces Work

An NFT marketplace lets users list tokens for sale and lets buyers purchase them. Behind the scenes it relies on the ERC-721 approval and transfer mechanics you have already learned.

The marketplace contract never owns your NFT - it just moves it when a sale executes.

The Approval Step

Before listing, the seller must approve the marketplace to move the token. Sellers usually call setApprovalForAll so one approval covers all their NFTs in a collection.

<code>// seller authorizes the marketplace
nft.setApprovalForAll(marketplaceAddress, true);</code>

Storing a Listing

The marketplace records each listing with the seller, the NFT contract, the token id, and the price.

<code>struct Listing {
    address seller;
    address nft;
    uint256 tokenId;
    uint256 price;
}
mapping(bytes32 => Listing) public listings;</code>

Creating a Listing

To list, the seller calls a function that verifies they own the token and that the marketplace is approved, then stores the listing.

<code>function list(address nft, uint256 tokenId, uint256 price) public {
    require(IERC721(nft).ownerOf(tokenId) == msg.sender, 'not owner');
    require(IERC721(nft).isApprovedForAll(msg.sender, address(this)), 'not approved');
    bytes32 id = keccak256(abi.encodePacked(nft, tokenId));
    listings[id] = Listing(msg.sender, nft, tokenId, price);
}</code>

Buying an NFT

A buyer sends the price as Ether. The marketplace transfers the NFT from seller to buyer using safeTransferFrom and forwards the payment.

<code>function buy(bytes32 id) public payable {
    Listing memory l = listings[id];
    require(msg.value >= l.price, 'insufficient payment');
    delete listings[id];
    IERC721(l.nft).safeTransferFrom(l.seller, msg.sender, l.tokenId);
    payable(l.seller).transfer(l.price);
}</code>

Delete Before Transfer

Notice the listing is deleted before external calls. This follows the checks-effects-interactions pattern and prevents reentrancy from buying the same NFT twice.

<code>delete listings[id]; // effect first
// then interactions (transfers)</code>

Marketplace Fees

Marketplaces typically take a percentage fee. The contract splits the payment between the seller and a fee recipient.

<code>uint256 public feeBps = 250; // 2.5%

uint256 fee = (l.price * feeBps) / 10000;
payable(feeRecipient).transfer(fee);
payable(l.seller).transfer(l.price - fee);</code>

Royalties (ERC-2981)

The ERC-2981 standard lets creators earn a royalty on secondary sales. A marketplace queries royaltyInfo and pays the creator their share.

<code>function royaltyInfo(uint256 tokenId, uint256 salePrice)
    external view returns (address receiver, uint256 royaltyAmount);</code>

Cancelling a Listing

Sellers should be able to remove their listing. Only the original seller may cancel.

<code>function cancel(bytes32 id) public {
    require(listings[id].seller == msg.sender, 'not seller');
    delete listings[id];
}</code>

Off-Chain Order Books

Large marketplaces like OpenSea avoid storing every listing on-chain. Instead sellers sign orders off-chain, and the contract only runs the transfer when a buyer submits a matching signed order. This saves enormous gas.

Security Considerations

Marketplaces are high-value targets. Guard against reentrancy, validate ownership and approvals at execution time, and never trust prices or addresses from untrusted input without checks.

Quick Check

Test your understanding of NFT marketplaces.

Recap

You learned how NFT marketplaces operate:

Sellers approve the marketplace, which then transfers on sale (no escrow needed)
Listings store seller, NFT, tokenId, and price
Buying transfers the NFT and forwards payment, deleting the listing first (checks-effects-interactions)
Fees and ERC-2981 royalties split the proceeds
Big marketplaces use off-chain signed orders to save gas

Frequently asked questions

Is the “Marketplaces” lesson free?

Yes — the full text of “Marketplaces” is free to read here on the web, and the Web3 & DApp Development Fundamentals course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web3 & DApp Development Fundamentals course, upgrade to CoddyKit PRO.

What will I learn in “Marketplaces”?

Listing and transfers. You practise Web3 & DApp Development Fundamentals with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Web3 & DApp Development Fundamentals?

No prior experience is required. Web3 & DApp Development Fundamentals on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Marketplaces” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Web3 & DApp Development Fundamentals lesson?

Yes. Every Web3 & DApp Development Fundamentals lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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