I advise that you send the trade first to your buyer, then they add the gold. There won't be any errors then. <3

I wouldn't advise users sending payments for items via the donation system. While I feel you have good intentions, other users might get on the defensive and say you're trying to scam them. Know what I mean? "You send the gold first and then I send the item!" sounds dodgy, even though I don't believe that's your intention.