What is the appropriate response when someone says “Thank you”? Most Americans would answer, “You’re welcome.” And in most situations that is the expected reply. But in some circumstances, the appropriate response to “Thank you” is another “Thank you.” When? After people have concluded a commercial transaction. The salesperson says "Thank you," and I reply, "Thank you." And that makes perfect sense. When one person gives another a gift, the recipient has given up nothing, and thus deserves no thanks. But voluntary commercial exchanges are generally mutually beneficial. Both sides gain, and this symmetry makes it sensible to have both parties give the same expression of gratitude.
(Giving credit where it’s due: I heard someone make the same point a long time ago, but I honestly can’t remember who. I think it might have been Walter Williams.)