Also another way: Pawn shop then add 30 to 40%. That will give you local market low end of value. Remember if you use gunbroker they are going to charge you to sell them and the buyer is going to have to pay shipping, which may result in a low valuation.
Try going to two different pawn shops to get an average offer between the two. Having worked the gun business buying guns for a gun store, we found that the pawn shops usually offered more than we did and had less of a mark-up as they had a higher turnover rate and thus more accurately reflected the going market value in the area.
I have used the gunbroker method also, especially for those rare, desirable, or out of the normal items as it gives you a better selection of comparables, but for common older items, you will be better served by the pawn shop method as for those older common items, some folks use GB simply to dump those guns for cash and it depresses the true valuation you would see in the local market.