Online selling etiquette
Sounds like your transaction was part online but local pick up. In this case you did it right by taking first offer to come through with payment.
If it were an online only transaction with shipping.. then I would give the first PM person dibs and let them know I have serious buyers in line (and to make a decision bunch amount of time or have to move on to next buyer).
Im helping my father sell his B body cars and this exact scenario comes up all the time. We’re inclined to want to hold vehicles for the first interested potential buyer but, realistically the first buyer with monetary means to close a sale is 100% the way to go.
So to recap: On FABO/FBBO first post gets dibs but, if selling an item that needs to be picked up locally (like an entire car) ... first buyer with cash in hand. Be overly communicative to manage everyone’s expectations.