Unhappy buyer .What to do ???

Used parts are used parts,and it,s a roll of the dice.If the buyer does not show a tooth pattern wear on the gears and you have used them with no problems,you owe ZERO.If he can not come up with the SO CALLED tech that looked at the gears,and his number,to talk to him,he COULD be scamming you.Was there some sort of noise,high pitched humming,noise on costing,if not,he has no leg to stand ,mrmopoartech