Feedback issue

Sold a preamp with remote.Remote worked all the time I had it.Buyer claims it doesnt and left me neutral FB.Now I am disappointed and wonder what I can do.I offered to send a universal remote.He says no to that.I will offer to buy unit back.Haven't heard back from that.I see it as his word vs. mine.What should happen next?Thanks,Bob
agree with Stereo5.

even if you two could have maybe agreed to split the cost of a new remote....that could have been done before they submitted feedback.