There are lots of fake Rolexs out there and some are almost indistinguishable from the real thing. I have bought a few before so heres a few pointers so you dont get stung.
1, If its unbelievably cheap, to good to be true it probably is!
2, Look for good 100% feeback, from UK sellers, preferably one with Rolex sales in their feedback.
3, If possible always pick up in person, if the seller dosent want you too, walk away.
4, If possible take to a Rolex dealer to check authenticity, the only real way is to look at the movement.
5, Always make sure the Rolex has all correct paperwork, and original sales receipt.
there are genuine sellers out there, but just be careful!!