"What can stop me copying this CD and selling the copies for the cost of postage?"

Well if the album is out-of-print and the company that issued it no longer exists and the performer that made it is dead, I'd say that there isn't a morality issue. (The issue of legality is only an issue if you believe what is legal is more important than what is moral.)

But rather than selling copies of the CD, I would simply digitise it and stick it on YouTube or soundcloud.

Or alternatively, one thing that's often occurred to me to do is to digitise an album and put it for sale on Bandcamp with all the money to be given to charity. That is, to reiterate, ONLY in the case of albums that are out-of-print, where the company that issued it no longer exists and the performers that made it are no longer with us. And, of course, where the album is so good that I think it is a genuine disservice to the world that that music can no longer be heard.