First let me say that I do not profess to be an expert and there can be no substitute for a detailed examination by a qualified watch maker.I have bought and sold a few Rolex watches and I am active a member of a few Rolex watch forums.
I must say that I have seen many fakes on Ebay and it is illegal to sell them.Ebay has strict policies about this and so does Rolex.The watches can be confiscated and legal action can take place.
There are three types of Rolex watches,the genuine Rolex (nothing more needed to be said),the fake Rolex (a counterfeit version passed as the real item) and what they call the homage version (not a fake as it will have no Rolex markings and possibly its own brand name.) Homage versions are not fakes but it obvious that they are copies of the genuine Rolex watch,whilst not illegal they are close to the boundary.
Another thing to remember is that so called high quality Italian copies/replicas are still fakes and most likely cheap rubbish marketed better so the criminal can make more money.
Buying a Rolex.
Keep this in mind when looking at a Rolex.These watches are very high quality and the fit and finish will be good,except for natural wear and tear.There are several general things to look for as a guide;
1. There is no such thing as a bargain.
2. Check the sellers feedback and look for many transactions dealing in watches and high end items,with good positive responses.
3. Sellers that state that the item was found,it was a gift or don't know if the item is genuine, WILL probably know that the item is a fake.
4. Pay by a safe traceable payment method.Ebay has its own guides on this.
5. If you buy from overseas you will need to pay duties and taxes.
6. Research and do your home work and join a forum so you can post pictures of the item.The members will be quite happy to guide you.
7. Ensure that the photos are of the actual item.Ask the seller to set the watch to a unique time,date and email you the pictures.
8. Look for paperwork,manuals and boxes as this can help the authenticity but certainly WONT guarantee it is genuine,as there have been quite a few fake boxes and paperwork scams around of late.
9. Take into account the age of the watch when purchasing as automatic (mechanical/perpetual) should be serviced by an Authorized dealer or at least a qualified watch maker every 5 (five) years.This can range from several hundred dollars to over one thousand dollars.
10. If your watch is sent to a Rolex service center for repairs and is not genuine,it can be confiscated.If it contains counterfeit parts Rolex may refuse to service/repair or even confiscate the parts.
11. Above all be careful and approach each sale with care.As stated before if it seems to be too good to be true then it probably is.
I hope the above can help as trying to go into the different aspects and what to look for would fill many pages as there are many models and versions,there is even is a quartz version (Oyster Quartz).Remember Rolex watches are quality items and have great resale.
BTW. Checkout my other Rolex guide.