Not gonna lie- as PB65stang said, I've had deals where I just got tired of dealing with the buyer. Not saying this is your case, but it does happen. I had a guy once ask me every question possible about a car I was selling- I answered honestly as always on every single one..... at least 5 times! He came and drove it.... 3 times with different people. He claimed he wanted it, said he'd take it 95% sure, but just wanted to make sure it was 'the right one'. I told several other potential buyers it was sold, which ended up being a mistake. The guy drug it out for almost 2 weeks, until finally I told him forget it, and sold it immediately to someone else with cash in hand. Since then, it's available until paid for. Again- not saying your situation is the same, but things like this do happen.
Working on old cars teaches us patience... and every curse word imaginable!