A buyer is committed once the bid has been placed and won. That's part and parcel of Ebay rules.

I understand what you mean about the 'buyer' being likely to retaliate with negative feedback. Personally, I would email Ebay and explain that the buyer won't actually pay and you're worried about negative feedback. Ask them for their advice and let them sort it out, if they will.

You have of course paid to advertise the watch on Ebay. Not a lot of money to advertise, more of an annoyance factor that somebody has messed you around. The 'buyer' should of course be responsible for reimbursing your advertising costs but the truth is you're very unlikely to recover those.

Email Ebay and explain the situation. If a non-paying buyer leaves negative feedback in retaliation ... well, it shouldn't be allowed. Ebay should be able to either prevent negative feedback in retaliation, or remove it from your rating should it happen.