One way to find out is to look at each of your components separately and hop on eBay for comparison. In some cases you can maximize what you can get for your rig by selling parts separately. However, there is the hassle of listing, shipping, fees etc.
I recently sold a 2600K and an Asus Z77 motherboard with 8GB ram for $350.
These would be average prices for your stuff:
i7-2600 - $150-$175
Motherboard - $50-$75
Ram - $20-$30
Graphics card - $50-$75
Case and PSU - $40-$50
Hard Drive - $30-$40
Total $340-$400 Approx.
Personally, if someone offered you around $350, I'd take it..