How the fuck is a shop supposed to establish who fucked up some BB threads. As the seller was aware it was a French BB, has used a French BB beforehand and advised the seller to do the same, I'm going to take a massive punt and say that the new owner most likely did the damage by trying to fit a non-French BB into the BB shell.
He took it to the bike shop, they removed the bb and then came along and told him the threading in the bb shell wasn't in great condition and needed sorting, he at no point tried to fit a non french threaded bb, as I wouldn't allow him to make a move so stupid
How the fuck is a shop supposed to establish who fucked up some BB threads. As the seller was aware it was a French BB, has used a French BB beforehand and advised the seller to do the same, I'm going to take a massive punt and say that the new owner most likely did the damage by trying to fit a non-French BB into the BB shell.