I've found that my best odds of finding firearms at investment level prices, or of negotiating prices down to investment level, are at pawn shops.
A typical pawn shop will only pay 1/3 of what they feel they can sell the gun for. So already they have minimal overhead into a gun, and can be talked waaaay down if you are waving cash around.
Many pawn shops are not specialized enough in firearms to research all models enough, particularly the obscure models, to make consistently appropriate valuations.
Because of the nature of pawnshops, they tend to want to turn inventory around quickly, to get back money that they have loaned out and subsequently lost if the owner does not pick up their item.
People who sell guns to pawnshops tend not to know the value or rarity of the gun they are selling, otherwise they would be somewhere else, or advertising in the paper.
I think back on my best deals, and they have all been in pawnshops.
Case in point, I bought a Walther PPK, an original WW2 PPK that after research I nailed down to a fourth version RSHA gun that was issued to a member of the SS. The pawn shop had it listed for about 20% of its actual value, because they looked it up under generic wartime PPK info in the blue book of gun values.
A few other guns I've bought in pawn shops, I'll try and remember what I paid vs what they were listed for
Springfield M6 scout survival gun, listed for $150 + tax, paid $125 out the door, I sold it for $150 like a complete dumbass...neither the pawnshop or me knew what we had. worth $700 or so last I looked.
RIA .45 GI model, 99% with 1 mag listed at $350+tax, paid $300 out the door, got $350 in trade value
Taurus PT99 98% with 2 mags, listed $375 plus tax, paid $300 out the door
Ruger MK1 bull barrel, 90% with 1 mag, listed at $175, paid $150 out the door
H&K USP .45, 98%, listed for $500 + tax, paid $400 out the door
Rem. m700 .270 ADL LNIB no scope, $275 OTD
Win. M70 stainless classic .300winmag, with leopold 2-10 VXII both 99% $600 out the door
Sometimes I get lucky at the local gun shop, but not as often as the pawn shop and usually only if its a consignment gun that the LGS doesn't set price on. Like my diamondback .38 2.5" for $400.
They can be an investment, but for the time and capitol you have to invest, you either have to buy looots of cheap guns and hope they outpace inflation,
or invest in large dollar firearms that get even more large dollar, again...outpacing inflation....and the storage costs.
Otherwise after your time and energy, you won't be making diddly squat on each gun, and your money would work much harder elsewhere.