Does it have a stable overclock? People love that.
I would try to post an ad in the local newspaper. You could link them to your website, which would have lots of pictures, and information on specs.
Otherwise, you could try Craigslist, and agree to let people come and examine the PC.
Is dust minimal, and is everything clean?
Is your graphics card the 1GB or 2GB version?
RAM information, CAS, modules?
Does your case have USB 3.0?
Which DVD drive, PSU, keyboard, and mouse?
What version of Windows do you have installed?
Full model numbers/information could give me a more accurate price.
Do you still have all manuals and documentation, including product boxes?
For the price, a knowledgeable person would pay around $700 to $750 for it.
A person with limited knowledge about building computers could give you around $800 to even $1000 for it.
Here is your build:
http://pcpartpicker.com/p/1DOMZ