Well like I said in my first reply, the person buying your PC is the one taking all the risk because for them there are many unknowns. People buy used to get a deal, quality comes second. (it's stupid but true)
If your RAM is 1600 ddr3 then it's $40 and your 1TB HDD is $40. Your GPU/CPU might be worth $100 together. This is why we've all been saying between $200-300. It will depend what your case looks like to if it's all scratched/dirty when people try to buy it they will want less.
However, if you "brand" your computer, you can get more for it. I was recently trying to sell an old 2006 MacBook Pro for my mom that had been gathering dust. I checked ebay to see what others were selling for and all were about $200-300. Then, I saw this one...this guy had a full page of info with stats/info and really made thing look impressive, even though it wasn't. He didn't lie/exaggerate about anything, but he just provided the info that people wanted that made them feel like this computer was good for them. He sold it for $650.
I think if you can find out what your computer is still capable of and post stats like "can play BF4 at high settings!" then people would be more likely to pay more for it. Have a few of the major played games in your description.
Another thing is that people really like having windows installed already. This doesn't mean you activate it for them, but just use Dban on your HDD then reinstall with your Win7 CD so when they turn on their PC they have all the basic drivers set up and major updates done so all they have to do is activate it. This is up to you, but it is a nice selling feature.
Last is to include everything you can with the computer. All the driver CDs, manuals and a cheap keyboard/mouse if you have it, just so they have options.
I think you could add an extra $100-200 on this way.