GameStop and iPhones with bad ESN numbers

I'm only asking this out of curiousity I don't actually have an iPhone with a bad esn but I see them all over eBay and Craigslist. From what I have gathered you cannot activate a sprint/verizon/att iPhone if the ESN number is bad. This would make the phone completely worthless yet I see them selling for hundreds of dollars. I've read about people flashing them to Cricket or a couple other carriers to get them to work but even that should drop the value down considerably.

Now, I assume people sell iPhones with bad ESN numbers to GameStop and whether they are stolen or attached to unpaid bills and it seems there is no way for the GameStop employee to tell without calling the number in to the carrier. This is where my question comes in. Without a doubt gamestop has purchased iPhones that were either stolen or on unpaid accounts. What is GameStop doing with these phones? I know they resell them and I know they send every phone traded in to be wiped clean but as far as I know that's not going to fix a bad ESN. How are they reselling these iPhones with bad ESN numbers? I know a corporation as big as GameStop wouldn't jump into this market without having this figured out. I'm guessing they have some kind of agreement with carriers to clean esn numbers so customers can activate GameStop bought iPhones. If GameStop can't clean the ESN number, then they are eating tons of money on the phones they are buying off people. If GameStop can clean the ESN number, then there should be a way for us all to do it.