The unethical part is that the OP made digital copies, so selling the tapes theoretically deprives the artist of a sale. Two copies now exist where only one did before.
However, given that I don't think commercial cassette tapes are even available for sales in stores anymore (I haven't seen any in years, at any rate), it's probably not a big deal.
First let me set something straight... the OP (Me) did not make digital copies. I am in the process of getting digitals (ie CD's). I dont have the time or money to try and digitally remaster the quality of the cassettes into something worthy hanging on to. I still have cassette players around the house but Id rather listen to them on my iPod while working or in the car.
The reason for this thread was to try and find out if anyone knew of places buying old cassettes.