I agree with rjseeney on that going to a D100 is not an upgrade. Always better off new than to think you are saving getting a used dSLR body.
The D90 is a much better choice.
If you are set on getting it anyway. I would check to see the shutter actuations and test to see if the sensor is needed to be cleaned or not. There are posts on checking your sensor for dustbunnies and the like. Even though it is used the $180 price is pretty low to me.