If you have really sent the check I would simply stop responding to the person, other than restating the date you sent the check. After a while you'll have the canceled check, and if you are later arrested for not paying for something you'll have an email from the guy stating what he felt he was owed and a canceled check to match it.
And I agree with G above... eBay has a number of perhaps well-meaning but nevertheless bad buyers and sellers. The buyer was dumb for sending a check for that much to a seller, and you should have thoroughly inspected what you were selling before listing it. In any case I'd call the cops too if if didn't have my money FedEx'd back. I wouldn't stop at the local police either... I'd be in the post office filing mail fraud charges as well.
But I would stop responding the the guy. If this gets into a larger legal mess you want fewer statements from yourself in writing, not more.