This is probably a question for FFL's more than anything but anyone who has insight would be helpful here.

When you evaluate what you should pay for a used weapon (custom built AR's are excluded because everyone thinks they have the most valuable carbine known to man!), what data source do you use? I have found "Blue Books" and "Black books" for guns much like for a car or truck, but I don't know how reliable something like that is.

In today's market, being so demand driven with all the first time buyers, manufacturing constraints on new weapons, and ammo crunches I want to make the best and most educated decisions when trying to expand my own collection. I always think i have a "feel" for what a pistol or bolt rifle is worth but that is not in any way scientific ad I was hoping to have a more mathematic approach.

Let me know what you think!

I usually go on GunBroker to shop used prices. Find the auctions that actually have bids on them, throw them on your watchlist, and it'll give you a good idea of what you can expect to pay.

not the worst strategy.. at least it would say that someone is willing to pay "X" amount for the weapon. If only you could tell if they are panic buyers or actual knowledge based buyers!

Start at Gun broker and maybe look at the EE on here.

I do check the EE but I find it's a similar situation to Armslist. No one is selling something to break even (most of the time) so it's still a bit of a guessing game. But it is probably more accurate than Armslist

Assuming you have a gunbroker account (I don't) you can search completed auctions.  Sort by number of bids in descending order (highest at the top).