Anything can be a collectible if two or more people want it. Demand is paramount. After that you can break it down into provenance and condition. Was it owned by a famous person like Elvis? Is it in remarkable condition for its age? Is it the first or last of its type? What makes it special in the eyes of a buyer. Does it have a famous name in it?
Collectors tend to be specialists. They collect one brand, or one model or even just one version of a model. Pigeon grade model 12s in small gauges for example. Winchester model 70 Safari models. They tend to ignore the common place items in average or worse condition. They want ones which look brand new. And they hate refinished or upgraded items. They consider them fake, a fraud or attempt to get them pay top dollar for a bogus item.
