I mean Person A makes an NFT and lists it for sale for $X USDC. Then Person B, who only has $Y USDC ($Y < $X), buys it using a pre-approved loan from Person C i

I mean Person A makes an NFT and lists it for sale for $X USDC. Then Person B, who only has $Y USDC ($Y < $X), buys it using a pre-approved loan from Person C in the amount of $X - $Y. If Person B repays the loan within T days, the lien is removed, so Person B owns it free and clear. Otherwise, the NFT is transferred to Person C. The pre-approved loan would just be a message signed by Person C, granting anyone permission to transfer a certain amount out of Person C's account if (in the same transaction) they also grant Person C a lien on that particular NFT. Person C's loan funds wouldn't be in an ordinary account, but rather in a smart contract which understands these liens and loan approval messages.