Depends on what it is for options on valuation aka well known or obscure item and gunbroker is usually high on pricing so area the sale would occur also affects value
I pay attention to gunbroker frequently. I also look at empire arms, liberty tree collectors, axis arms, and a few other gun sites to see what they're selling for
It entirely depends on the gun itself. With old milsurps, there are so many different things that make one more valuable (or collectible) than others, so its important to know the differences. Gunboards.com is a really good option, they are incredibly knowledgeable on most all of the milsurps.