Fake Legacy cards will cause sales on Theros/Born of the Gods to tank. They will also be responsible for the sharp decline in attendance at Standard events.

If people reselling their fakes becomes problematic, I expect stores with excellent reputations will inspect any cards that they purchase second hand. So places like Star City Games really stand to profit from the counterfeits, while the counterfeits will allow newbies to break into the Legacy format. On MTGS, Galspanic has posted a set of the counterfeits along with a set of real cards. I had to ask him which of the sets were the real cards, because they really do look good if you're not running the usual tests on them.