What happens when an offer is accepted
The listing goes under offer, the other offers close and the agreed terms are recorded.
When a seller accepts an offer, the listing is marked under offer and every other open offer on that car is closed automatically. Those buyers are told the car is no longer available.
Only one offer can ever be accepted. If two acceptances arrive at the same moment, the first one wins and the second is refused.
A record of what was agreed
At the moment of acceptance we save a fixed copy of the deal: the car, the agreed price and the terms both sides accepted. It cannot be edited later, and we use it if there is ever a dispute.