As a quick guide, I would take a look at the current prices for each component, add an assembly fee, and subtract 10-15% in depreciation.
Video Card: ~$100
CPU Cooler: ~$60
System total: ~$935
Plus Assembly: ~$65
A case could run from $50 to $800, but that system looks like a $50-$80 case. PSU looks like about 550w, probably $60-70. If you have a BD drive add $100, $150 if its a burner. For a CD or DVD drive, I would only add about $10-20, even if it will burn. They are so cheap nowadays that I would pay almost nothing for them
Max total: ~$1120
I would start bidding at about $850-$900, depending on the value of the case, PSU and disk drives.
Good luck selling.