The technical term is "wardrobing" and it is the primary form of return fraud. I am pretty sure it happens all the time on these fancy kids clothes, such as today -- the guest buys the Easter/Christmas clothes, removes the tags, has photos taken in them, and returns the clothes ("Oh, the clothes didn't fit and all the tags just happened to fall off accidentally"). Of course, if the clothing still looks like it's in new condition, has never been washed (the DPCI tag is usually a good telltale sign), and I can still sell it, then I suppose why not sell it again? After all, it's already been touched by the grubby hands of Bangledeshi children, dockyard workers, and everyone else down the supply chain, it's not really different if a kid wore it for 30 minutes as well so long as someone else will pay full price for it.
But mostly I see junkies bringing in stolen merchandise, returning it for a gift card (usually no receipt but occasionally with stolen receipts -- a bunch of dirty, wadded up receipts with all different cards, which I do as a gift card anyway) and then either buying beer or -- more commonly -- simply leaving and taking the gift card to a pawn shop to get cash in order to buy drugs. They are pretty easy to spot as they are all dirty and probably homeless and reek of alcohol or are starting to get the shakes from needing a fix. Sometimes they steal things themselves but criminal rings are more common -- that way a different person returns it than stole it, making it harder to track.
If we really want to crack down on return fraud, we should add RFID tags to every item. That way each and every item in the store has a different code. We could scan an item and see exactly when (or even if) it was purchased somewhere. With clothes, we would have to do like Bloomingdale's or Macy's and put giant tags on them that must still be attached. There are definitely more secure ways to do business and it wouldn't really inconvenience the legitimate guests with real issues, only the criminals -- whether the homeless junkie type or the suburban housewife coupon scammer. Those shouldn't be the "guests" we are trying to attract.