Legal Question in Business Law in California

You didn't say why you were uncomfortable with the buyer, so you haven't said anything that would cause me to believe that you have legal grounds to back out of the contract. From the limited information you provided, it does sound like the buyer has legitimate grounds for a lawsuit unless you allow him to pick up the item(s) that he paid for.

While I tend to agree with Mr. Starrett, I'd add that eBay transactions are governed by eBay's own rules and policies, which may be binding upon users, so I'd inquire with eBay's dispute resolution service, giving them all the facts as accurately and fairly as possible, and then weigh their advice. Try to stay within eBay dispute-resolution guidelines.