Is to buy stuff on ebay, then pay with forged check. Seller deposits check assuming his bank can tell if check is good. Bank misses forgery and credits his account in good faith. Feeling safe, seller ships goods. Second bank (the one the check is supposed to be from) detects forgery (as much as a couple weeks later), and denies payment. Transaction rolls back to seller's bank which then decrements seller's account and optionally charges bad check fee.

Goods gone, money gone, you're screwed.

This happened to a guy selling a Mac laptop and he managed to track the guy and get him arrested but more often they get away.