Its amazing to me what people think their used PC's are worth...
I see this ALL the time on Craigslist...
In fact I just blew chocolate milk through my nose while reading an ad for a pentium 3 with less than 512Mg ram that someone wanted $200 for..
Complete PC Setup Linux Ubuntu 10.04
You have to remember that the PC your selling has no reciept, no warranty and nowhere to go...
Theres no upgrade potential without spending more than its worth.
Its also burdened with Vista
Youd be lucky to find someone willing to pay over $150