We actually have laws here that state that an item must be on sale at a certain price for a set period, before it may be marked as a sale priced item. Traders also have to state if an item was previously sold at a sale Price, and what that price was ... basically this means that putting an item "On Sale" multiple times is not very effective :)
You have to be careful when shopping, but you absolutely can buy a *retail* license *used*/second-hand from a previous owner who is willing to transfer the license to you. So if you are in the UK where (apparently) new OEM licenses are not readily available, give eBay a shot. Don't buy from anybody selling what looks like large quantities of "new, shrinkwrapped" Windows 7 retail boxes at too-good-to-be-true prices, because they are almost surely counterfeit. But buying a clearly used retail copy from someone who only has a single copy to sell and who took the time to take their own unique pictures (not stock photography) should work out fine.
If you have Vista on the machine already and seek out used retail *upgrade* copies, that's probably even better because 1) hardly anybody bothers trying to counterfeit the *upgrade* boxes when doing the "full install" boxes is so much more lucrative, so chances are way better that you are buying valid product, and 2) the upgrade versions routinely sell for less than the full versions anyway.
When you get the discs and product key, and go to install and activate it, since it is a used copy, it is very possible that the on-line activation procedure will complain. All you need to do is call the phone number that it presents you with, and explain to the representative that you are transferring this copy of Windows 7 from one computer to another, that you have already removed it from the computer it was originally installed on, and that you are only installing this copy onto one computer. They won't give you any grief. They will generate a phone-based activation code for you which they will read back to you, and which you will enter into the dialog box that is on the screen. Voila.
-- Nathan
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