But this time, I bought the one that was identified as "New," because this discussion makes me wonder to what extent I'm being complicit in unethical behavior when I buy a used CD.
There is a difference though. If you own the original media (even bought second hand) then you have not broken the law. In this case, it is the person who has ripped it (kept a copy) and then sold the original that is breaking the law. I agree there is a concern that you might be complicit but it is hard to be certain. Many of the used or hard to find CD dealers on Amazon sell thousands of CD's a month - to me they look bonafide sellers (like used books) but like you I never checked - great question!