I would steer clear of any keys being sold on ebay by themselves. They may be legit, but more than likely not. More than likely they are from TechNet or MSDN.
If you are going to buy from ebay, I would look for retail or upgrade versions in unopened boxes only.
Edit: You might also check craiglist in your area. I lucked out. Bought a Windows 7 Pro Retail for $130! Again, in person only with craiglist and then unopened boxes.