I've sold CPUs on eBay before, and it's not so bad. In fact, I should be selling an i7-860 (+ motherboard + RAM + cooler) soon.
Whenever I sell PC components on eBay, I usually state their usage. For example, I don't really ever OC my CPUs, but for some reason, I always buy the K-series processors. So, I'll state exactly that. I may even try to denote that the CPU was kept at a good temperature at all times given things like an aftermarket cooler, good case airflow, good ambient temperatures, etc. For water cooling stuff, I normally mention the lack of dyes in my water, what biocide I used, etc.
Ultimately, the descriptions along with your feedback should try and instill confidence that the buyer isn't getting a piece of junk. Arguably, the more description you give, the safer the buyer can feel, because providing a false description is grounds for a refund.