The person I bought my plane from told me he had a guy coming who was pretty confident he would be buy the plane.
That person would be coming in on a Thurs. If I got there before he did, first buyer takes it. The other guy didn't have a deposit or firm commitment. When I showed up and did the deal, the seller told me that he was happy I bought it, because the other guy was going to put the plane in a flight school.
Still, first $$ or purchase agreement or deposit usually takes it or has first right of refusal..