You have an agreement with a shop owner, that you can buy stuff from him wholesale at a discount.
You then hang around in the shop (Maybe in disguise) waiting for customers who are about to buy stuff.
Instead of letting them get on and buy stuff, you accost them, and suggest they let you act as a middle man, and you'll buy it wholesale from the shop keeper, and donate the difference to charity, after you take your cut.
Sounds shady doesn't it. And the shopkeeper is being forced to do it until he finds out about the scam.