Broaden the concept of "value exchanged for an NFT"

[tinker-michaelj]

Background

The final spec for HIP-18 (Custom Hedera Token Service Fees) defines the value exchanged for an NFT as follows:

Say a CryptoTransfer includes an NftTransfer with sender account 0.0.X. Then the value exchanged for this sent NFT is all fungible value---hbar and HTS token units---that is credited to 0.0.X in the same CryptoTransfer.

‼️ The network only collects royalties from the value exchanged for an NFT. We have recently seen an NFT marketplace accidentally bypass royalties by using two transactions for an NFT resale.

1. First, an approved spender sent an NFT to a buyer, receiving the buyer's funds in an escrow account.
2. Second, the escrow account sent part of the buyer's funds to the original seller.

Because the approved spender in #1 was not the sender, the network did not collect royalties.

Proposal

We suggest broadening the concept of value exchanged for an NFT to include all fungible value (other than fees) that is credited to any account in a CryptoTransfer that exchanges an NFT.