Some of this might seem flaming obvious but sometimes you have to point out the obvious.

Firstly, I always look at their feedback and where they are from, if the item is coming from China, then it probably isnt genuine.

Check what other items the other person is selling, if they are selling other Tiffany pieces at the same time then they are probably not genuine. If they are clever they will only sell one piece at a time so check their recent activity by looking at their feedback.

Also, when looking at their feedback, look at the items they have been buying and selling recently. They make be selling a Tiffany piece that they recently bought and not satisfied with its authenticity.

Always be aware of newbies and people with very low feedback selling high priced items. And dont fall for a sob story of their reason for selling their Tiffany & Co item as that can be a trap.

It never hurts to ask if they have a recepit for it.

I hope some of this information has been helpful to you. I personally have bought several items and have only been caught out once and thats when I was new to the ebay game! I have also taken my items into Tiffany stores to have them cleaned, they will not clean it if its not genuine and they do closely inspect the items too, so if in doubt try it out!!