Here is the idea...
Lets say that weapon merchant currently does not want to buy any new firearms (already bought the desired quota).
Why not have ability, that if you buy a gun from him, you gain ability to sell another gun in its place?
Hey, Mr. Merchant, can I barter this gun for that SMG?
How much to I need to pay extra for that?
That would be really cool.
It would encourage bartering, which fits setting nicely.