In reply to danrock101:
People lie. It's a shocker.
A few years ago, we sold a washing machine in the local newspaper. Old, but in working order. Forty quid.
A woman phoned up, got the address, turned up and bought it.
Later the same day, she phoned up quite irate, saying that it didn't work and she wanted her money back. I politely acquiesced, apologising profusely. She went on a bit, to be honest, but I continued to apologise and sure enough, when they brought it back, it was dead as a doornail.
Couple of days later, down the pub, I related the tale. Oh, said one of my mates, they've nicked the PCB. I'd not actually heard of one of these before, but he said that it was a known scam, so the same night I slid the lid off the machine - and sure enough, the Printed Circuit Board which makes the machine work was missing. They'd essentially taken it away, stolen the damn thing for their own machine and then brought it back for a refund. I had no contact number, nothing.
True story. Turned me into the man I am today.
Are you absolutely sure the buyer is trying to give you your own watch back, or might it be the one he bought previously, changed the battery on (voiding the warranty) and then swapped out for yours? It happens, you know.
Terms and conditions of sale protect BOTH sides. There's only so much you can / have to promise on a private sale.
Martin