Is it safe to buy Adidas Superstars on eBay

Selling on ebay ad threatens .. ??

A friend of our family has sold brand new and unworn shoes from the brand Adidas Superstar, the very exquisite and rare special model Safety, on ebay!

Payment, checkout - all good! Unfortunately, the shoes were a tad too small for the buyer, so he resold the shoes on ebay !!

Now our friend got a message yesterday from the first buyer that the second buyer wants to be of the opinion that the shoes are not original, but from somewhere from the cheap counterfeit market, i.e. from Turkey, Poland, etc.

Our friend knows that the shoes are 100% GENUINE !! He had bought 2 pairs of the said shoes from a "scene shop" in Leipzig in 2009 and has the now "old shoes" he had worn in the basement. The second, double pair ended up for sale on ebay as a collector's item.

Unfortunately, there is no longer an invoice for both pairs of shoes - and the shop in Leipzig - where said shoes were bought - has probably not existed for a year either ... 🙄

The brand new pair sold was sold in the original box!

Now our question:

How can our friend best behave and protect himself legally from the outset when threatened with a complaint? Unfortunately, he does NOT have legal expenses insurance and so he might have to pay a lawyer himself dearly first ...?!?

Does anyone have an idea or a legal basis on how best to proceed in such a case ...?!?

The first buyer would of course refer to our friend in the case of an advertisement - which would be logical.

Does anyone have experience in such a case and have a good initial idea ready ...?!?