Like mdocod and thor220 said I agree that closed loop watercooling isn't a good option compared to the air cooling counterparts. The fans that come with them in general are a bit noisy which adds to the cost of buying quality/quiet fans. CM's Nepton series seem to have a quiet pump but they aren't on the lower side of the cost spectrum.
I can also agree with thor220 that used computer components bought from Amazon (Marketplace) is pretty nasty. The prices seem a bit higher also. That being said I never said I assume that everyone is a computer expert, and suggested you contact the seller before buying anything used. Plus cleaning something is not a thing that only "computer experts" or "experts" should know about it. It's a common thing that kids are told over and over again when parents make them clean their room and other things.
What kind of "bad cleaning" can you do to a piece of metal that hinders the performance of it? Cleaning it which a metal scrub? I honestly don't know and it would be interesting to see what kind of performance changes happen with things like leaving the old paste on for a long time before cleaning, cleaning with different materials (paper towel, tissue, sponge, etc), different solutions.