Big
I hear what you are saying and know it is a dissipointment to think you have a car sold and it doesn't happen after waiting. Anybody can back out of a deal for any reason at any time if no money has exchanged hands.
But what I do in that case, is just edit the ad to say "sale pending"and keep it online. That way you don't miss any potential buyers while waiting for the guy to make his move. If the buyer requests you to remove the ad, ask for a non refundable deposit. He will either get in gear or take his chances someone else gets a good deal.