I would think that likely was an honest mistake.  When they are buying tons and tons  of scrap, they're not going to be able to keep track of every piece which was in every pile.    If you are buying pieces, don't leave them laying in someone else's pile. 

I buy quite a bit of parts on auctions.  I always move my stuff to piles I can keep track of, and don't leave high value items laying around. I have never had any trouble losing anything yet.