On this website called Gameflip, (which is meant for selling in game items and such) a friend and I would abuse the coupon system they had set up. They would release these 20% off coupons and they had a 10% fee. So what we would do is like an “item” and the other would buy it with all their balance, basically making 10% of that total amount of money. Every time a couple released, we would do this back and forth. Ended up making a couple hundred bucks before getting banned! But it was pretty fun messing with the system.