Idea!
Since it reasonable to argue that something rare and hard-to-manufacture, like a firearm where the technology is new, would be expensive, and since it is unreasonable to gimp the gunslinger class by not providing them with a gun, or by discouraging their use in the world, et cetera, and since a solution of "it is hard to sell guns" or "you'll never be able to sell this specific item for 20% value" is not very good game design, how about this:
The starter gun(s) that gunslingers get is a hand-me-down, or a hack job. Something that the gunslinger themselves have trained with and repaired and worked on over and over -- maybe even made. They know the gun(s) inside and out, and therefore can use it without issue....but in the hands of anyone else it is a broken and useless piece of scrap metal, worth maybe 10gp on the best day.