there is a thing called a blue book of gun values, you can buy and ion it will be a ton of information on most guns
next you can simply, look up what model /models firearms he has, and look to see what others are asking for them on web sites that sell used guns, just remember that what you see in ASKING price is seldom the price things sell for, subtract about 20% or more and your more on target with what they really sell for
I bought and sold guns for a LONG time
any decent gun shop near you can also put a value to them, but just as said above, they will NOT offer you what there worth as they have to sell then, to they will normally buy them for a LOT less than you can sell them for, but some times the hassle of selling isn't worth the added money you get, all the more so if there are a LOT of guns and you want them gone in a fast one sale deal!
and NEVER take the word of just ONE gun shop, odds are sadly MOST will NOT tell you the truth on ALL things,
add on's to most things, scopes, stocks and or?? seldom bring even HALF what they were new, so don't expect them to add a lot of value to things, just saying!
but if you made a list I am sure folks here would throw some numbers at you to get a clue on what there worth!
heck never know some might even want to buy them off you LOL
I know I have too many guns now, but I am still tempted at times to add more ! I think its a sickness I have! HAHA!