$180 is kinda low, but getting it from a private source is taking a huge chance. I certainly wouldn't sell my D50 to finance the purchase. If you insist on getting a d100, go to a reliable source like KEH. They've got used D100's for around $235 in excellent condition.
Also, you should consider why you think you need an upgrade. And again, aside from speed, you're really gaining nothing by getting a D100. Minimum, you should look towards a D200 for an affordable pro type body, or the D80 for a more affordable consumer body.