Unhappy buyer .What to do ???

I would have to say that if it were me buying the gears knowing they were used i would hope the seller would be honest and from what ive seen you were but i would also expect the worst as i have been on both sides before . I have bought stuff sight unseen that someone was honest about and i got better than i expected and i have bought stuff that was no where near how it was shown or described . Any time I buy anything used or new sight unseen as soon as it arrives i look it over closely to make sure its the right thing and its in as described condition .
if its not ,that person will hear from me within a couple days not months.
Also correct me if im wrong but Every rear end ive ever done feels a little different turning forwards than backwards due to the fact that the way the gears are cut they are the strongest and are designed to contact better when the vehicle is moving forwards , the contact does change slightly when turning in reverse and in a way are working against the cut pattern .
J.m.o. but feel doesnt cut it, proper measurements and verifying contact patterns . gear and bearing conditions do . I say unless he comes up with accurate measurements and pictures of their measurement methods and condition of the rear that better look exactly as you remember it I would say sorry about your luck , its yours now so use it sell it scrap it or what ever not my problem .