I have to agree - I think it is really silly for 2K to allow multiple purchases for limited collectibles.
It baffles me that this would occur for something that is out of 15 as well - surely they know that most buying multiples are doing so to make a profit and screw real fans over.
And like you, I really struggled when I thought I had missed out because I have been collecting Bioshock memorabilia since before the release of the first one. I just kept thinking that I'd missed out to people that were just trying to make a quick buck (though I'm sure that's not true - I was just upset). At least only one has gone up on eBay so far. Hopefully the others are all being loved.
Anyway, like I said before - the signed book is probably the centerpiece of my collection now. It will most certainly be cherished, worshiped...
