Hunh? Someone hands you two dimes when you leave? What's the point of
buying the transfer and selling it back, rather than just taking a
transfer and handing it in when you exit, as they do in Seattle? In
Seattle, riding downtown is free, so you pay when boarding outside
downtown, or when exiting outside downtown -- and if you're passing
through and coming out the other side, you get a "transfer" when you
board and pay, and hand that in instead of paying when you exit.

If they gave you an exit transfer for no charge, you could be a non-local
passenger and ask for the transfer and give it to someone else who could
then exit for free (not at your entry station, probably).

By charging you for it when you enter, they guarantee that you'll only buy
it if (a) you're going to use it to exit, or (b) someone will buy it from
you (which makes no sense).