I just reviewed the NFT Item code and I can't figure out how getgems.io is interacting with it.

- Which opcode are they using to interact with NFT Item?
- How are they parsing the message from the bidder? (I tried to make an offer for an NFT and got an error.)
- From end to end, if I make an offer, which receive functions will be triggered by the NFT Item? Or is it just a front-end error check?