Test new tyres before auction as new tyres?

Hello, I auctioned 4 new tyres at ebay in an auction. There is still dealer’s or manufacturer’s warranty on the tyres (I still have the original invoice). DOT is 2811, was also indicated in the auction. Since I wanted to check if everything is fine with the tyres before the sale (since I always exclude the exchange, withdrawal, liability for defects in property), I mounted the tyres on rims and drove about 100 km with them. There is an imbalance, loud running noise and steering wheel flaps, which was not the case. Of course, the tyres still have the original new tyre profile height of 8-9 mm. I sold the tyres as new with the description “Ware is checked before shipping”. The selling price was 323,- Euro + 40,- Euro shipping. The currently cheapest new tyre supplier at ebay and Google/Shopping is 500,- Euro for this tyre set. The buyer complains now that a tyre has an imbalance (was with not me!), so the tyres have already mounted on rims! And criticizes that the tyres are “not new” but have at least 1000 km down. How would you act? Would you take the tyres back? (The problem is that the shipping costs are 40,- Euro each time) I offered him that he can claim the imbalance with the manufacturer, but for a return I have no interest, especially since he must have already mounted the tyres on a rim to determine an imbalance can.