Don't be quick to point fingers or start yelling either, the seller may not have known much about GBB pistols either.
I sold a KJW m9 to someone on here which I thought worked fine. He knew more about pistols than me, and apparently there were a few parts that had to be replaced, so I helped pay for them. Both parties left contented and with +1 feedback.
I change primaries like other people change socks.