Ebay is awesome, I've never bought any GW stuff from GW or their stores all my models are from Ebay. You can get most models for 33-50% off, if you're willing to repair and repaint used ones. New In Box items though cost almost as much as from a store.
I doubt you'll get things like the Baneblade for cheap, if you buy single vehicles on Ebay they go for a lot. You'll get a better deal if you buy odd lots, like a few vehicles and some squads in one package. Also, if you are looking to raise some cash, Grey Knights and Noise Marines sell for a lot on Ebay right now.
As for good deals, I got 2 Tornados, 2 Razorbacks, and 20+ well painted Ultramarines, including 5 Davastators, for about $100 total. That was a good deal to me. Terminators for $4-5 a piece is good too, at least compared to retail. The Dread I got for about $31 was nice too, because here they cost about $55 at a store. Pocket Rulebooks are about $16, for those who don't want to pay $50 for the fancy one at a store.
I think GW would nuke ebay if they could ...