If you have an Intel system buy the full version. (In the end it will cost you about $300-400 to do it the right way). If you have a non-Intel system FCP6 is the end of the line.
FWIW – Most of the time people selling older version (for the upgrade price no less) are passing on a copy that they have upgraded, this is a form of piracy. When you upgrade the older version are not up for grabs, they are tied to the upgrade.
While there is nothing that prevents this from taking place because Apple has very lose registration methods, and we want them to keep it that way, the way to do that is to be honest.
IMO what makes FCS so much better is all the other programs, especially compressor and DVDSP. Then of course Motion, Color and STP. So there is a lot more that more features to the editing software.