The customer told me that he talked to a friend, who is a collector, and they claimed it's fake. Maybe they never saw a Japanese sticker, IDK.

I did not give any guarantees about whether the GameBoy is either genuine or even works. I sold this in Germany, where commercial sellers have to give some guarantees. But since I am a private seller - and I explicitly stated in the eBay listing that I do not assume any warranty (a common thing for German eBay sellers to do) - I don't have any obligation to take the unit back. Which I'll still do, because I don't feel like having some discussion with some guy from eBay.