I've had very good luck with the folders off eBay. Of the ones that I've bought, about seven or eight have been dogs. All of them have had to be serviced. The one in the worst condition -- literally falling apart -- was from Russia. Only one has had leaks in the bellows.

But I agree with Chris' suggestion that you get a guarantee from the seller. A sticky shutter can be serviced. Physical damage isn't worth fixing, and patching bellows is an exercise in frustration.