The problem wouldn't be selling your iPhone, but rather the apps, music files, purchased videos, etc. loaded on it. The answer to that is simple: wipe the phone first.

It has always been an issue really. Anyone who bought a computer with a Windows operating system and other software (like Office) was buying non-transferable licenses for all that software. You never owned it, you simply bought the right to use it for yourself. If you gave away or sold that computer with that software on it to someone else you were technically breaking the law. It's software piracy.