one problem with the shops doing the exchange like this is that you can't be sure what they do with the other set.
each manufactorer's policy differs in different industries. some will take the old sets back from retailers, recond them, and send them back out, some will take back, recond and sell to staff.
the problem is with small retailers who exchange a set for you, which bears minor or cosmetic faults, and then passes that to the next person with the hope that the next person either doesn't see it, or doesn't make a fuss.