[Fred]

Perhaps statements like the following would clarify things: "anyone willing to pay the full asking price will trump any negotiations in progress".

However, if you already agreed to a negotiated price then that would be poor form to go back on your word for a higher offer.

If I'm in the middle of a negotiation and the seller says someone is going to pay full price, then I'd understand if the seller stops the negotiation and sells the card to someone willing to pay full price. And, I wouldn't turn around and tell the seller I'd be willing to pay full price because someone else was interested in it. That would just seem wrong. If I wanted it that bad, then I'd just pony up the cash right away.