If you buy a SWU or other upgrade cert off eBay/Craig's List/etc, UA can do things like shut down your UA account, cancel your tickets, etc.
I would recommend against it. There were theories a while back that UA had people posting SWU sales on eBay, in an attempt to catch people buying them. Believe DL has done the same thing.
If you have no status, there is a good possibility that you'll get a Tens of Dollars (ToD) upgrade. Check out
http://www.flyertalk.com/forum/unite...iscussion.html for more info about that process.