Having sold tiffany before, I would say that above 95% of it being sold on ebay is fake..
Too, fm1234 is 100% correct about tiffany pursuing fakes in an aggressive manner, even more so that louis vuitton.
You can buy excellent fakes on ebay, which will include the correct markings, boxes, polishing cloths, tissue paper and authenticity cards. You can even bring these item to the nearest tiffany store and most of the employees there will not be able to tell that they are fake...some items, especially the braclets, will even be of the correct weight...
However if you go to the official tiffany website and really look at the 'enlarged view' of their items, you will notice subtle differences from the ones you see on ebay..especially pay close attention to the lobster clasps and hooks..the 925 markings will be a deep and crisp and clean stamp. This is one of the most common things that people take for granted with tiffany,they assume that since it has a tiffany ny, or tiffany co. or 925 marking that it is authentic. But what you need to look for is the quality and especially the location of these specific markings...Just take a look at the tiffany website and as I said use the enlarged view of the piece that yu are viewing and you will see what I am talking about...
Also if you check the site frequently, you can save yourself some money on selected items, as they do mark them down from time to time...