@juiceme yes those black blotches are also very common!
@aspergerguy good point, maybe that is what happened. I just thought it was strange and wondered if there was a more complex reason, like the back panels are removed by some people for normal use and so if there is one being sold without the panel it probably means it's stolen... or something even more sinister!

Good tip about the SIM pins and modern 3in1 cards.