This type of thing is always going to happen. Any store that lets customers exchange or refund devices without adequate checks in place is always going to get screwed. The HN store I used to manage generally wouldn't allow people to exchange an item once opened, or allow change of mind refunds for these very reasons.
The few times we did allow it, we nearly always got screwed. If you let them exchange it, you've still got to sell the product after you get it back, and with computer margins like they are, to get rid of a device as "used" will require dropping your pants (especially when your stock costs have the Gerry Tax applied to them).
Way, way too easy to crack open an external, swap the drive out with an old one and return it if your store policies involve returning things without adequate checks, and not having staff on the ball enough to be all over these potential issues. Even then, it's still too easy to get screwed these days. After you've taken a direct loss in the thousands for this type of things, your interest in letting people return things because they changed their mind seriously drops. Especially once that seal is broken.
