Hi Matt,
The IT175D isn't a terribly highly sought after bike I'm afraid - hence their value, especially in these slightly quiet times isn't that high. They had a few mechanical quirks too.
If it's in the condition you say it's in, then I'd have thought the $1500 to $1600 you were thinking of offering would be a fair offer.
Remember, a bike is only worth what someone's prepared to pay for it....