Well, you are both wrong. If you have opened the retail package, or used the software on the computer it was pre-installed on, you automatically agreed to the terms of the license. The license states that you must purchase a new copy for each computer, and that the license is non transferable.
I believe that such licensing practices are wrong and should be illegal. This does not alter the fact that if you agreed to the terms of the license, you are bound by it.
So while you can sell an un-opened retail copy, you cannot legally sell an OEM copy, nor can you legally sell a retail copy after opening the box.
Doesn't anyone pay any attention to the license agreement, or the sticker on the box that states "by breaking this seal, you agree to the terms of the license"?