My guess is that it's an 83 760. size 58 or 61.
$500 is expensive. Not a give away. $350-$400 would be a bit more reasonable, but if I didn't have a new frame coming, I'd consider $500 for this. It's got suntour superb, which is, welll, superb stuff.
If this is the 760, it would be a fantastic Spokane road bike --- think road/dirt/easy trails/etc.
ad: http://spokane.craigslist.org/bik/793769973.html -- the seller, in colbert, has a couple other interesting bikes.
from original catalog: http://www.vintage-trek.com/images/trek/1983/83Trek15.jpg