I collect Remington and Smith & Wesson firearms that were standard production models that only enjoyed short production runs. For instance, the 5" S&W 629-3 Classic DX, which was only made in that barrel length in 1994 or the Remington XP-100R, the repeater version of the XP-100 that was only made in 1998, the last year of XP production. Other Remingtons would be the Model Six pump rifle, an embellished version of the 7600, and the 870 Competition, a single-shot pump trap shotgun with a gas-assisted recoil reduction system, which both were only made for five years back in the 1980s. Another is the Model 700BDL made from 1997 through 2001 as those rifles had engraved receivers and magazine floorplates.
True one-of-a-kinds and special-run items often cost too much to buy and their higher resale value limits the number of potential buyers you might find of you want to sell them. But I think S&Ws like pre-IL, pre-MIM 686 PowerPorts and the various Classic hunter versions of several S&W models that were produced in smallish numbers for a company like S&W make affordable potential collector pieces.
Accurately predicting resale value for anything is difficult but one can only make his/her best guess. For example, one option or feature can make all the difference in the world in a car's value. Rear disc brakes on a 1969 Camaro Z28 increase the car's value tremendously because only 200 had them and the 1969 Camaro was made for a year and a half, as the 1970 didn't come out until spring of '70. A folding rear seat is even more rare - if I recall correctly, only 30 were made with that seemingly meaningless option.
But buying, collecting and shooting any nice firearm is fun so if it later becomes valuable, that's just icing on an already delicious cake.
Ed