I'd forget the idea of selling it "overclocked" because generally someone who buys a pre-built system on ebay probably doesn't know what they're doing, much less what "overclocked" means, or how to troubleshoot problems if something happens to the BIOS settings between when you sell the computer and when they receive it. If the OC for some reason doesn't maintain via transit or the first startup when the buyer receives the computer, you'll be responsible for selling them a system that's "slower" than advertised, which would be a pain.
Keep in mind that people who sell computers for profit on ebay have fairly large businesses and they can make and sell things in bulk, which is where their profit comes in (some systems might sell for cost, some might sell for a small amount above cost).
Not many people are going to want to pay for shipping on a home-built computer with no warranties (like tech-support, etc. for the entire system--like they'd get through dell for example).
You might want to try to sell it locally.
Advice for the general sale:
People interested in buying this computer probably won't be interested in the components individually, but rather as a system as a whole--so I'd ditch the "overclockability" motherboard, and just buy a basic board. The Q6600 might be more appealing to the average buyer because it's a quad-core, but on the other hand, the average buyer probably won't know what to do with it...