Just to update, I purchased the Windows 7 Licence from the eBay link, and the code worked perfectly, and came with the COA sticker, so all above board. I can't comment if the disc that came with it worked or not, as I used a different OEM disc.
Also, I just had a look through the link to the MS site, and if you read this paragraph
Generally speaking, in the case of OEM or OEM System Builder software, the licence may not be transferred to a new or different PC. However, the entire used PC, including the software media, manuals and Certificate of Authenticity, may be transferred to another end user along with the software licence rights.
I believe this is what the eBay sellers are doing. Basically they are selling the whole PC, along with the licence. If you look at the advert they state the PC can be collected from their location if desired.
Anyway, just thought I'd let everyone know it worked.