Unhappy buyer .What to do ???

I sold it for $375.00 plus shipping .He said the guy that checked it out told him it was okay to use as a spare ,but didnt turn as easily one was as it did the other ,so that meant the gears were bad or something.He told me this guy has been doing gear setups for decades .We talked through pm's before he bought it and i asked him if there was anything he wanted me to check or verify that i hadnt mentioned ,and i would do my best to get him what he needed.I made it clear that i am no expert in gear sets ,but the wear pattern looks good and it did ...

I don't see anything to complain about because it was a used part, even his guy said it is ok to use as a spare so it can be installed and used. That being the case I would say he should have bought it new if he expected a new part.