I buy tons of every day products on Ebay but recently I have received cosmetics that were unsused but very old and dried up or just smelled funny. You should ask the seller for the expiration date. While they say it is "fresh" and "unopened" they never tell you how long it was sitting on the shelves.
Other products I have purchased for 50% off include my printer cartridges for my HP computer, razor blades, shampoo, designer-labeled shoes, designer jeans, etc. Know your size before you buy. Some sellers will even accept returns.
Caveat: Do NOT buy designer bags - most of them are counterfeit.