OEM (original equipment manufacturer) is not to be sold on it's own, first of all. That is illegal. It is to be sold with other computer equipment. Second, it usually comes with a greatly reduced warranty (not always though). The reason is that the VAR you bought it from is supposed to provide the warranty and that is why the product is cheaper. Lastly, you get no tech support for an OEM product from the manufacturer. Again, this is the job of the VAR.
I buy OEM products for my builds and then take the responsibility of supporting my clients. I have recourse through my suppliers though. You won't. I do suggest where at all possible, unless you don't mind taking the "risk", that you purchase retail parts so that you can get the best warranty and support.