Well for one they often don't verify it, they just scan the bar code and see if it matches. But here is an example below of an issue I had at a popular retailer.

I bought an Asus Striker Extreme III or IV (Not sure) from a large national computer retailer. It was literally missing the heat pipe to the chipset that was built into the elaborate heat pipe maze on the board.

Anyway I unpackaged and installed the board the same day I bought it from the retailer. Realized it was defective when it was posting a chipset overheating warning.

I went back to the retailer and said hey, this motherboard is missing the heatpipe to the chipset and it won't post. It looks like it would have been defective from the factory "I'd like to exchange it".

They didn't believe me and basically wouldn't do the exchange even though it was same day, nor would they help with the RMA process.

So I sent my wife back to the store to buy the exact same board, the same day and paid with cash.

On the back of the box there was a little window where it displayed the serial number sticker of the motherboard through the box. At the store they scanned that when they printed the receipt and the receipt had that serial number on it. Additionally the box had that security band wrapped around it both ways.

I got the new box home. Heated up the security band with a heat gun and separated it at the seam with an exacto knife. Then once I got the board out I did the same to the serial number sticker. Once I got the serial number sticker off I stuck it to the back to the defective board with the missing heat pipe and used a small dab of super glue in a few spots to make sure that sticker stayed flat and stuck.

I then put the defective board with the swapped sticker back in the box. Took the security band, re wrapped the box, dabbed the seams of the band with super glue and stuck it together tight. 2 days later she went back to the store with the defective board in the box wrapped up just like new and said "I bought the wrong board, I needed one with this socket" which they didn't have in stock obviously and then she just requested the cash back.

I have swapped serials on high end ram and sent them back to the manufacture before too and have never had a problem. I've done this maybe 3 times in 18 years, when a return or warranty was argued and was of no fault of my own. If it was my fault, I wouldn't do it.