As he said, how often do people really care if they get the manual or not when buying a used system? They might care if you have or don't have the driver CDs, but a lot of the time that's not even the case. I'd say, get OEM CPU if you plan on getting an aftermarket HSF, retail mobo just to get all the cables, retail GPU if you need any of the extra cables, OEM HDD and optical drive since the cables come with the mobo, OEM mouse and keyboard. That's about how I did it. The manual the mobo came with was crap, and I never used the GPU manual...
I think a system w/o manuals would be as easy to sell as one with manuals. I've sold countless used systems, a couple I had the driver CDs for and gave that to them, but mostly I didn't. And I never had a single manual to give them and they didn't mind.