All I can tell you is to follow the eBay rules TO THE LETTER. Make sure all emails and transactions are handled within the eBay system. Take an hour and go read all of it.
Do NOT accept a non-major bank cashiers check UNDER ANY CIRCUMSTANCES. For example, you can accept Wells Fargo, BofA, Chase, Wachovia, etc., but only if you can go to a local branch and cash it. Do NOT deposit it in your account. Do not accept postal money orders. Do not accept any check that is not for the exact same amount of the deal. Run like crazy if the buyer asks you to refund him any money.
You should also get a copy of his ID as part of the transaction. Have him photograph it and email it to you, scan it, whatever.
Do a bill of sale that is complete and binding.
Report the sale to your local DMV.
And never under-report the sale price of the vehicle.