A couple of years ago, I bought a CD of XP Pro for £15, to sort out a friend's PC. When it arrived, it was very obviously an illegal copy. It went back with a stiff letter and my money was refunded.
I then bought one on eBay for £60. That was legit, with the COA and the hologram etc. It was still in an unopened wrapper. It came with a "piece of hardware" - a 64MB USB memory stick. Apparently, that made it legal in MS eyes. It installed fine, anyway, and the lady in question is still happily using it.
More recently, there were ads all around the web for XP and Office 2003 etc for something just under £20. They, too, were bogus.
So if it's on eBay for under £50-ish, don't worry if you don't win it.