I am not knowledgeable, but Google helps in searching.
https://www.google.com/search?sxsrf=ACYBGNQ3SyWIguqY-Zt37B0WV5h6hP9RvA%3A1569280477937&source=hp&ei=3VGJXcuLNtX_-gS68K3ABw&q=buying+selling+used+gun&oq=buying+selling+used+gun&gs_l=psy-ab.3..0i22i30.6049.23143..23813...4.0..0.155.2969.13j15......0....1..gws-wiz.....10..35i362i39j35i39j0j0i131j0i13i30j0i8i13i30j33i22i29i30.lYLgCJN_rBY&ved=0ahUKEwiL7r-riejkAhXVv54KHTp4C3gQ4dUDCAc&uact=5
Use BING...it’s better.