Value of Winchester Model 50 12 Gauge Shotgun?

I recently inherited a Winchester 12 gauge shotgun and was pondering selling it to fund another firearm purchase but not sure of the reasonable value of it and thought I'd ask around. From the serial # it appears to be from 1959, pics below, thoughts?:

http://img809.imageshack.us/img809/9799/mg2508.jpg
http://img217.imageshack.us/img217/3251/mg2509.jpg
http://img818.imageshack.us/img818/2295/mg2510.jpg
http://img10.imageshack.us/img10/1410/mg2511x.jpg
http://img11.imageshack.us/img11/449/mg2512i.jpg
http://img26.imageshack.us/img26/3033/mg2513.jpg
http://img651.imageshack.us/img651/4633/mg2515.jpg
http://img195.imageshack.us/img195/2562/mg2516.jpg
http://img850.imageshack.us/img850/9296/mg2517.jpg
http://img706.imageshack.us/img706/6499/mg2518d.jpg
http://img7.imageshack.us/img7/474/mg2519.jpg

from the gun digest book of values it spans from 150 in poor condition up into the 400/500 range for v.g./excellent condition in standard grade. add 50/100 for skeet / trap grade guns.

my take is these prices are what you would expect a dealer to sell for, not what a dealer will give for it

How does one determine the 'grade' of the gun?