Ebay is a great place for a bargain, but just be sure that you know what is and isn't included for the money. A classic example is warranties on watches.
If you buy a watch from your high street jeweller, it will come with a warranty, usually from the manufacturer itself. The reason for that is that the jeweller is an authorised distributor, and so can offer a warranty on behalf of Gucci, D&G, Seiko, whoever.
When you buy from Ebay, you are very, very rarely buying from someone who is an authorised distributor. Those tiny minority who are authorised tend to be the higher-priced of the sellers i.e. sell for the same, or nearly the same, as they do in their bricks-and-mortar shops.
So, when you see a listing that says 'warranty included' you need to ask the following questions - "Who is the warrantly with, how long is it for, what does it cover and how to I go about making a claim if I have an issue?"
Some sellers offer their own personal warranty - much along the lines of a manufacturers, if the watch turns out to be faulty in the first year/2 years, then they'll replace/refund. This generally does not cover batteries or any diamante stones that come loose (no different from that genuine D&G warranty).
In the main, what most/many sellers are trying to do is convince you that you get a warranty with a watch when what you actually get is A WARRANTY CARD. Having the card gives you no rights whatsoever unless it's issued by an authorised distributor. It doesn't even guarantee that the watch is genuine - it's possible to buy watch boxes and paperwork off ebay for a few pounds, so has no practical value.
By all means look for a watch that has all the right paperwork, but don't use that as a guarantee of authenticity and certainly DON'T be fooled into thinking that a warranty card = your watch is under warranty. As ever with Ebay, buy from a seller with high positive feedback and use Paypal for maximum protection.
It's a trade off - if you want it cheap from Ebay, then the price you pay is the lack of a manufacturer's warranty. If you want that warranty, then the price you pay is the high street price - make an informed decision on what's best for you.