I bought a lot of E100GX (200+ 35mm rolls) from ebay over the last many years, generally for under $2/roll, and stuffed it in the fridge and freezer. I have a very long term project I'd like to shoot all on the same film, and learned my lesson when E100SW went away. I figured I'd want to have a lifetime supply of the stuff in case Kodak every stopped making it.
Well, now they stopped making it. I was clever, right? Except I can't shake the feeling that in my own small way I helped bring about its demise. After all, I never in my life bought a single brand new roll of E100GX! The whole "beauty of the free market" thing is great until one of those dips in demand makes Kodak throw in the towel entirely on something.
For those buying film on ebay, don't forget the somewhat hidden costs: Shipping costs, plus you really need to waste a roll from each purchase to test it out, since no amount of cost savings are worth having important images you thought you captured not really be there after all. So one roll plus processing costs from each purchase goes to waste. Which is why I only buy big quantities of film. There seems little point in winning an auction of one or two rolls. On the bright side, nearly everything I've bought has turned out fine. Some really ancient Tri-X 4x5 was a little fogged, but no surprise there.
The 4x5 Polaroid films are not good deals now, nor were the last couple of hundreds rolls of fresh E100GX 35mm that Adorama sold recently. But there are plenty of good deals to be had on more mundane stuff, if you shop carefully.