An interesting topic - there was an excellent article in Stereophile (I believe) a while back talking about copyright law and how it applied to music. My recollection was that the laws allow for a lot less than most of us think they do, but I don't remember the specifics. I'll have to dig through my old issues to see if I can find the article.
It's an interesting topic because there are several scales to evaluate it on - legality, fairness, common sense, etc. I'll admit that I would have just assumed that it was illegal to buy a CD, make a copy, and then sell the original, but I certainly don't know that it is based on my reading of the laws.
It's interesting to me that it is mentioned several times in this thread that the RIAA isn't worried about the single copy given to a friend, that they're focused on stopping the mass duplication. One scan of the ebay CD listings shows any number of people who openly are selling 1-1000 CDs because they have "ripped all the CDs to my computer and don't need the CDs anymore". I'm sure enforcement costs $$$, but if somebody is worried about this practice, they could certainly mount enough of a charge to scare people into at least not admitting in writing of what they're doing.
On the fairness scale, I'm not sure it's all that clear. At what point is one depriving the label and/or artist money? Is having a copy of a CD for which you don't own the original the equivalent of having stolen goods? If there was flat out zero chance you were ever going to pay a penny for a CD, have you deprived anybody of revenue by having the copy?
It seems like the scale is infinitely granular and everbody decides for themselves where right stops and wrong begins. Personally, I have a couple thousand CDs and I have never downloaded a song, legally or otherwise, off the internet. But I can certainly see lots of scenarios where people would and it wouldn't deprive anybody of a penny of revenue, because it wasn't ever going to be spent anyway. That doesn't necessarily make it legal or fair.
Regardless of what is legal or fair, I think the constant implication that the complete underpinnings of recorded music's woes is illegal copying is insulting. Has there ever been a bigger tidal wave to a major industry that was so unprepared for, and so poorly responded to as digital music and advances in technology? Clinging to the notion that you can charge $12.99 for a copy of Heart's Greatest Hits in 2006 and that you're going to be able to legislate your way to enforcing it, while wondering why sales are down is plain comical.