Marketplace Basics

A marketplace does not need users' private keys. It needs a standard way to move the NFT and the payment in one settled flow. On NEAR that is usually a combination of NEP-171 + a marketplace contract.

Two common patterns

Approvals (NEP-178). The seller calls nft_approve on the NFT contract, naming the marketplace and, optionally, a minimum price in a message. The buyer calls marketplace.buy(token_id). The marketplace uses the approval to call nft_transfer (or nft_transfer_from style transfer) to the buyer and pays the seller. 1 yoctoNEAR is still required on the NFT transfer, so the marketplace must use a full-access key for that account — typically the marketplace contract itself signs, because a contract call can attach a deposit from the contract account or from the buyer.

nft_transfer_call. The seller (or an escrow) transfers the NFT to the marketplace contract with a message. The marketplace implements nft_on_transfer, keeps the token listed, and later transfers it to the buyer when buy is paid. This is simpler to reason about (the marketplace holds the NFT) but means the seller temporarily gives up ownership.

A sketch of buy

#[payable]
pub fn buy(&mut self, nft_contract: AccountId, token_id: TokenId) -> Promise {
    let listing = self.listings.get(&(nft_contract.clone(), token_id.clone()))
        .expect("not listed");
    require!(env::attached_deposit() >= listing.price, "Attach the price");
 
    // Pay the seller, then ask the NFT contract to transfer to the buyer.
    Promise::new(listing.seller).transfer(listing.price).then(
        ext_nft::ext(nft_contract)
            .with_attached_deposit(NearToken::from_yoctonear(1))
            .with_static_gas(Gas::from_tgas(20))
            .nft_transfer(env::predecessor_account_id(), token_id, None, None)
    )
}

The exact nft_transfer arguments follow NEP-171 (receiver_id, token_id, optional approval_id, optional memo). In production you also:

• Refund extra deposit
• Clear the listing in a callback if the NFT transfer fails (promises are async; do not assume the transfer succeeded)
• Support paying in a NEP-141 token with ft_transfer_call instead of attached NEAR
• Take a fee by splitting the payment

Trust

The NFT contract is the source of truth for ownership (nft_token). A marketplace you do not trust can fail to pay you if you transferred the NFT into it first. Approvals keep ownership with the seller until the sale. Always read the marketplace contract, and start with testnet.

The diploma contract is not marketplace-friendly on purpose: it panics on nft_transfer, so a diploma cannot be listed or sold.

Exercise

Draw the sequence for an approval-based sale (seller, marketplace, NFT contract, buyer) in three or four steps. Mark which step is a view call and which steps are change calls.