That's not the one on Phoenix CL: http://phoenix.craigslist.org/cph/bik/1604218179.html Someone run out and grab it!

According to the S/N, it's a 1981 610 in 24".

Looks like it was marketed as an '82 614, however. The frame isn't quite as sought after as the one I thought it was (yours only has 531 in the main tubes), but it's still worth the money you paid. The crankset alone, if in good condition, sells for $50-$75.

http://www.vintage-trek.com/images/trek/Trek3pg6.jpg

EDIT: Beat to the punch.