This story is an everyday occurrence in the store where I work! Our damn air mattresses have a policy that was created just for them.
Every Friday and Saturday, we sell bunches of them, then every Sunday night and Monday 90% of the sold mattresses return to the store. And by now, the custys know the rules... MOST of them will say they are damaged (even as they claim they never opened the box!).
We've had to start opening the boxes of every returned mattress, because the customers have a new trick. They open the box, remove the mattress, and insert other items, then tape up the box. The tape jobs look just like brand new unopened boxes!
Its gotten to where even if the person has the receipt, we have to open the box to be sure we're actually getting a mattress back!