I use Fjestad's Blue Book of Gun Values to determine the value of a gun. Same thing your LGS uses most likely since it has pretty much always been an industry standard. It doesn't matter to me what someone wants to get or tries to get on Gunbroker or even what someone else may have paid for it on Gunbroker. The few that I have bought there for a reasonable price ended up being "Not As Described"
Unless we're talking high dollar collector type guns, I would just search the trader for similar guns and see what they are priced at , keep in mind many items on there right now are being advertised at higher than normal prices, most are sitting there as a result, you can search through the Old Ads part of it to see what many actually sold for , I don't like dealing with paperwork, and prefer to sell and buy meeting the other person face to face.