What's the catch with the cheap listings on Amazon?

I’ve recently discovered Amazon.com and I am in love. The prices seem to be pretty reasonable and there is a great deal of variety. One thing I worry about though is the very cheap prices listed from other retailers. I ordered a case for my ds lite as well as a car charger. The car charger cost me about fifty cents. I figure if what comes is crap, that’s okay because it was only fifty cents. However, I see a lot of these really cheap listings and it makes me wonder if I would be getting any kind of quality by ordering these. Naturally, I turn to fluther for guidance. Can I trust these really cheap listings?