I believe you need to file in your state.
I am not a lawyer, but I am fairly sure you have to file in HIS state.
There are plenty of good books on small claim courts.
Also, make sure you SNAIL MAIL him CERTIFIED your demands first so he can't deny receiving emails. I once lost a case because the person denied receiving my email (and he claimed to be religious always quoting the bible every time I saw him).
But since it looks like he could have commited fraud (even on a minor level rarely prosocuted) I would think he would settle fast to keep it out of the courts.
---------- Post added at 11:21 PM ---------- Previous post was at 11:03 PM ----------
What if the buyer is in a different country - I had 3 sales not completed on Sedo last year totaling $XX,XXX -
Even in the US, $XX,XXX sounds like it would be above the limits for most legally-informal small claims court and you would have to hire a lawyer.
Often the threats or filing is enought to get people to pay.
But unless the bidder screws up as in this case, the bidder can always claim that it was a mistake or misunderstanding. I once bid on eBay $99.99 for a book I was willing to pay $9.99 for, but fortunately no one else bid. But if the price was bid up to $50 I could have legally got out by honestly claiming it was a mistake.