Scammers go to big stores and fill a shopping cart with absolutely everything, from DVD players to sports equipment. They line up and pay for their purchases like anyone would. But outside the store, the person who paid gives the receipt to his partner. The partner then goes into the store and fills a cart with exactly the same products.
When they're done, they walk straight to the exit, skipping the till. They try and walk out at the same time as a bunch of other people. That way, if the security guard stops him, he can show them the receipt and say he already paid, and perhaps it was somebody else who set off the alarm. Soon after, the scammers will return all the products they paid for, leaving them with thousands of dollars of free equipment. Many times, they will take these products and sell them for cash.