When I buy gadgets, I check at the store to make sure that all the items (including accessories and manuals) are in the box, and that they're in good condition.
What the OP mentioned is a scam that's run on a lot of retail stores, which has led to them telling clerks to check serial numbers on hardware, etc., when returns and exchanges are made. They don't do it with unopened merchandise, though. And scammers know that they just need to get the weight right and re-shrink wrap an item to get it past a clerk. Sometimes they're inside jobs, too. In other cases, scammers will sub an old item for a new one.
There also have been reports of subbing scams done on merchandise that's delivered via UPS, FedEx, etc., with bricks or whatever being used by scammers to create fake weight for merchandise that's been stolen.