Originally Posted by Julian Yep, that's how I originally got Illustrator. I bought it from a friend who upgraded to a new version.

This is likely NOT legal, because to have obtained the upgrade they have to be a licensed owner of the version that you bought. In other words, you could also later take the original version and upgrade it, which would then create two copies of the same software with two owners without having paid the software company.

As for buying the software second-hand, some terms of service contracts say "no", others do not limit you. Since TOS' have never been really tested in courts, it's possible that a TOS that limits transferrence would be considered invalid. Or it could be considered valid because you agreed to it when you installed the package. Hard to be positive, but check the TOS first to see if that will throw up any roadblocks.