I was wondering about this.
In this day of global supply chain, what happens when you return a defective item to the store? It seems hard to imagine that it is worth while for (say) Walmart to send a broken Barbie back to the vendor. On the other hand, one would think that the vendor would want to look at the issue, possibly for failure analysis or verification of the defect. And while Barbies are obviously of nominal value, what about something like an air conditioner that could cost several hundred dollars, but nonetheless be a cheaply made commodity item from someplace in China.
Is there a general answer? Are the stores just credited for the bad item and the item discarded? If so, it would seem to create opportunity for abuse.
This was brought about in part when I purchased two air conditioners. The store scanned the serial numbers, but I noticed that neither unit had a SN on the actual device, only the box (admittedly there may have been one somewhere inside, but there was not one where the consumer could see it). I returned one as defective, but realized later that I had put it in the wrong box, and the store scanned the box, thus crediting me for the non-defective one. For me it was of no consequence, but strictly speaking their records now show that they received a different unit than the one they actually did.
I realized it later because the boxes for each were left in their respective rooms, and when I got home the room with the good one was the room without a box still there; I had carried it downstairs and sent someone else up to get the box for me.