I guess I'll be contrarian here, and advise you to buy it and shoot it. There are a lot of Series 80 GCNM's out there, and it's not likely to take off in value any time soon.

Right now, at a LGS, there are three used Series 80's that are going from $800-$900. All with boxes and 2 mags. Colt has been producing Series 80's longer than Series 70's.

The Series 70 GCNM brings a premium, because the mid-late 1980's models were sometimes not of the highest quality owing to the Colt v. UAW labor problems. The Series 70's seem to have better fit/finish than the Series 80's from the era. The only down side to the Series 70, is the collet bushing which is considered a weak point, and this was discontinued shortly after the Series 80 was introduced.

Buy it; shoot it! It's what Dr. Browning would prescribe.