I would rather someone sell, swap, or give away the cards I send if that means they will go to someone who really will keep them, than to have them just toss them in the trash. That is, for official PC. They were written to total strangers so I wouldn’t be writing anything I mind being shared. As for personal correspondence I always thought that the proper method of disposal is complete destruction.
Re-using cards by peeling off or covering the original message/address is just crass.