Use eBay's escrow service. Or have them wire transfer the money to your bank account.
I've bought two cars on eBay, both of them wire transfer - both times I arranged the shipping - all I asked the seller to do was to be there when the car was loaded - and that's all you should do.
You should also edit the ads to indicate your choices. Do not accept any kind of check, unless the buyer wants to wait 14 banking days to clear them, even if "Certified", "Cashiers", etc.