Not so fast. The problem is

you need to prove that you DID send it in mail. If you cann't proved a proof, you are in fault.

However, if that guy wants to file a complaint to ebay, ebay will charge $20 service fee. Therefore if the money involved is not much,it doesn't make much sense for him to do so.

That's why I always pay by credit card because I know I will be protected. I don't buy from someone who doesn't accept paypal/credit card.