You could ALWAYS have someone with 2 accounts, or friends that get together and overinflate items.
person A says to person B:
I have a rare fig, I'll put it on ebay for about 30% more than what it's worht. You buy it, and I'll reimburse you. I'll only lose a small % to ebay, and I'll post what it sold for all over the net giving a false sense of what that items value truly is, sell it a month later, maybe get an extra 20 or 30% for it, and make out in the long run.