I have been buying and selling on ebay for nearly 2 decades and generally found that 5% of transactions are bad, for whatever reasons.
In recent months I have had 3-4 "bad" sales where buyers made stupid complaints about perfectly good items, but I try to err on the side of complete descriptions and disclosure.
That said, just recently I made a bad call on a small transaction with a Geekhacker and I will owe him a refund.
**** happens, but ebay's feedback system, for all its faults (eliminating the option of leaving feedback on bad buyers?) is a workable solution.