If it's a lefty it's from the '70s at the earliest, right? Great period for rock-solid bird guns, but not a rare antique. Only the first few years of production (began late 1950) seem to get a premium for age, to the right person. It's built better than current production, on the same schematic, and people who bought new Express models are always looking for the old parts like the metal trigger group to gussy up their cheap guns.
The 12 gauge 870 is ubiquitous, and aside from top-end special trap models with figured stocks and the like, sells in the range already quoted. This is where condition makes all the difference, so lacking pictures it's really difficult to make a determination. If it was hunted with, chances are it was dunked in the drink a few times, and once you start taking it apart you might find 40-year-old mud under the fore-end; clean and oil it nicely before showing it to any potential buyer.
The LH is an attractive feature to a small percentage of people, but know that it gives up one of the great features of the gun: barrel inter-changeability. The left-hand port means the barrel tang position is reversed, so you can only use the more limited selection of LH barrels through the years. A long fixed full-choked bbl is not desired by most shooters these days.
