I thought debit/credit card technology was being used these days instead of stamps. Not in SC?
It's not hard to abuse the debit card system. The person doing the selling sets a price - say they have $200 available in benefits, and they are willing to sell them for $150 in cash. The buyer gives the seller the cash, in exchange for the SNAP card and PIN. When the buyer is done, they return the SNAP card to the seller. Cashiers do not ask for secondary ID when you're using a SNAP card.

I'm aware of several loose "co-ops" in my area that do this in order to obtain cash for gasoline, utility bills, and the like. Illegal as hell, but not at all secret.