Brokers and authorized minters

Two helpers show up around most collections: brokers and minters.

A broker is a go-between. A seller posts a sell offer. A buyer posts a buy offer. The broker puts the two together in one transaction.

If the buyer offered more than the seller asked, the broker can keep the difference as its fee. A marketplace can act as the broker.

The broker never holds the NFT. It goes straight from the seller to the buyer. The broker never holds the buyer's money beforehand either. Everything settles in the one transaction, or nothing does.

An authorized minter is a second account that is allowed to mint for an issuer. The NFTs it mints carry the issuer's address in their ID. The transfer fees go to the issuer.

A separate working account can then do the minting. An issuer can have one authorized minter at a time, or none.

Fixed supply. An issuer can prove that a collection is finished. It removes its minter, then disables every key on the issuer account. No one can mint with that issuer again. The cost: no one can collect that issuer's transfer fees either.

Brokered mode. NFTokenAcceptOffer with both NFTokenSellOffer and NFTokenBuyOffer set. With only one of them, the sale is direct.

• Both offers must name the same NFTokenID.
• NFTokenBrokerFee is the amount the broker keeps. The seller must still receive at least the sell offer's Amount before the transfer fee applies.
• The buyer is debited first. The broker is credited before the seller.
• The sender of the transaction cannot own either offer.

A common flow from xrpl.org: the creator's sell offer sets the broker as Destination. Bidders set the broker as Destination on their buy offers. The broker picks a bid and completes the sale. As a Destination, the broker can then cancel the losing bids, which returns each bidder's 0.2 XRP reserve.

Authorized minter. The issuer sends AccountSet with SetFlag 10 (asfAuthorizedNFTokenMinter) and the minter's address in NFTokenMinter. The address is stored in the issuer's AccountRoot. To remove it, send AccountSet with ClearFlag 10 and no NFTokenMinter field.

The minter sends NFTokenMint with Issuer set to the issuing account. Without the authorization, the mint fails with tecNO_PERMISSION. The minted NFT lands on the minter's page.

The issuer's MintedNFTokens field counts every NFT minted by it or for it. BurnedNFTokens counts how many of those were burned.

Fixed supply steps from xrpl.org:

1. Fund a new issuer account.
2. Authorize an operational account as minter.
3. Mint the collection from the operational account.
4. Remove the minter.
5. Blackhole the issuer.

Where the art lives. The ledger stores only the URI. xrpl.org lists three choices for the payload:

• A data: URL inside the URI, when the data is under 256 bytes.
• Decentralized storage such as IPFS or Arweave.
• A web server the issuer runs.

Related: Collecting safely.