Almost none of the weapons used in movies are sold off after the movie is done.
There are a variety of sources around the US that rent weapons & accessories for the entertainment industry. Stembridge was by far the largest before they went out of business.
I have occasionaly rented weapons to the movie/TV industry for 25 yrs. I'm one of the smaller sources for this and primarily only work in Mo, Kan, Colo, Wyo, New Mex & Utah. Since there are not a lot of movies filmed in those states I don't do a lot.
I also rent weapons to the WW 2 & Viet Nam reenactors.
Sarge