1. You are in the right to keep the deposit. As others have mentioned.
2. the moral aspect. Sometimes doing what is right isn't exactly the same as what you are entitled to. So I would keep $200-$250 and return the rest to the buyer.
3. Someone also mentioned asking for proof of car repair etc... that is an interesting idea. perhaps another alternative could be
if the buyer can show proof that the car needs mechanical repairs then you could either return the $500 or again keep a portion etc..
really it all comes down to what your time is worth. I'm not sure i would take time to deal with getting proof, continuing discussions etc. with this buyer. I would just make the decision now, deduct what I feel is fair and return the rest, thus avoiding future hassles with this individual. but i would not keep all of the deposit, and would likely return most of it... again really depends on what deals your've passed up and effort/time put into prepping the bike.