Its not so much 'how much you paid for it 2 years ago', as it is 'how much you can buy with the same money today'.
For $1200 you can build a hell of a machine. I don't have alot of time to post, otherwise I'd PM you a build as an example. But I've posted pictures of builds on these forums a few times before, maybe try to search if you have time. You also didn't list monitor, keyboard, mouse, etc. in your OP. So I assumed those were not included. Shipping a monitor 2nd hand would be risky also.
Anyways, it comes down to that there isn't much of a market for used computers. They are such a personal device, and poeple tend to neglect them most of the time. Also factoring in how fast new products come out, and come out for the same price I should add. It just doesn't translate that well into resale. I would suggest selling it for $1000 tops. But probably more like $800 if you want to sell it quickly. It has to be cheap enough that someone who is looking at possibly building their own machine with quality components would stop and think about the huge value. Anyways, I didn't mean to steal your sale thread. So I apologize for that. Good luck in your sale, and may I suggest post it up at some of the computer focused forums.
www.Hardforum.com has a decent amount of selling and trading, though there is a online seller rating system they usually hold keen on. You might want to check it out.
I built my system 2 years ago for $1300. I have a Q9450, 4gb Crucial Ballistix, 8800GT, 750GB HD, 780i nForce mobo, and a Antec Nine Hundred case. (Plus all the little things, dvd, SB sound card, Heat sink for the CPU, 750W PSU)