I tend to buy antique and vintage jewelry and I've had wonderful luck on eBay. You do have to have some knowledge, because you get the best buys from sellers who don't. I look for someone who might have gotten a few pieces of jewelry in a lot of something else and just wants to get rid of the jewelry so he can concentrate on the something that's his specialty. Also, new sellers looking to build up feedback sometimes offer excellent bargains.
I've had almost everything I've bought on eBay either appraised or at least eyeballed by a reputable jeweler. Everything I've bought that I knew was fine jewelry turned out to be worth much more than I paid for it. For me, sitting at my computer and finding a piece I love beats running all over the place trying to track down something at an antique store or house sale.
The worst experience I've had was at an anchor store jewelry counter at the local mall. This was back before I'd done much research and I had no idea how much things cost. The piece was beautiful but very, very overpriced. I don't even bother with department store jewelry areas any more.

