[G4] MS for Mac 2004 license issue

[Technophobic_Tom at comcast.net]

There are lots of sales for this on eBay right now because users 
bought it to get the 2008 for free and then recover some of the $150 
for the Student/Teacher version (the only one I'm interested in).  To 
get the freebie, buyers have to send in the UPC off the box. The 2004 
license states that it can only be used at one household location. 
So, if it has been installed by the original buyer, using one of the 
three certificate codes, can they legally sell it? If they haven't 
used any of the codes and haven't installed it but have used the UPC 
to get the upgrade, can I buy it and then legally install it? How do 
those codes work? Are they all unique? When one number is registered, 
does MS know what the other two are and thereby can prevent me from 
installing it?

You get the picture. What advise can you offer?
-- 
T.T.