My 2 cents. As has been said, this second-hand issue is a very grey area.
There are second-hand stores and plenty of avenues on the web. I've not heard big labels complaining much about second-hand sales (but I'm not particularly listening either). I'm sure there are lots of people who buy physical albums knowing full well they may just rip it and sell the pristine copy. And with the price of downloads getting higher at many stores, you might as well buy the album, rip then sell it if the price is right ... you might actually get the album cheaper than buying the download.
I think the real legal issues are actually very similar to commercial software. You own the right to use the software, you don't own the program. And selling the software can only be done if the company allows the registration to change ownership ... which means you no longer have the right to use the program if sold because it is then registered to someone else. This is of course enforced to some degree with various copy protections schemes available to programmers. With music the enforcement is pretty much impossible, so is largely ignored.
I don't have any stated restrictions on my DataObscura site about selling albums you have bought (and keeping a ripped copy), and not sure I've seen one on any other web label either. It would be unenforceable, and a bit of a put-off to some buyers I'd imagine.
Especially with selling downloads, which are so easily shared, you have to trust the integrity of the buyer. Some of them certainly do respect a kind of unspoken law on these things. Others clearly don't.