75 GBP for the CPU, 60 for Vista, 50 for RAM (based on ebuyer prices), etc. Add them up. Subtract 20% because the buyer doesn' t get to customize it. Add 10% for your effort to build and install the software.
Honestly, I wouldn't buy it. The 9500GT is pointless in games, and the SLI MB doesn't help because the PSU is very small and 9500GT SLI is silly anyway. You need to find a buyer who doesn't care about games, or find a gamer and agree to replace the video card and PSU when you negotiate the price.