A friend of mine found some shoes she liked in the right size at the wrong price in one store, and in the wrong size at the right price in another. She bought both and "returned" the wrong sized ones to the higher priced store.

This really seems like a solved problem in Australia. Once you leave the store with your purchase your reciept is stamped. If a stamped reciept is coming out of the store it's a simple check of the date and time, which can be verified by a checout-person. Ezpz. The biggest issue for theft that I saw was putting other items inside the box of a larger item. Say you buy a lawn mower, walk the box around the store, put some other items inside that box, pay for the lawn mower.

Returns are mandated here though and I guess the US is anti-consumer in a lot of ways so companies won't really be forced to find solutions that don't explicitly benefit them.