There are discussion boards similar to FreeRepublic for specific brands. S&W, Colt, Ruger, AR, AK, and such.
Most have buy & sell boards. Generally need to join to post but that’s usually quite easy. I like using those as you have a targeted, interested and knowledgeable audience.
That’s a great idea!! That’s why I ask these types of questions on FR because of the depth of knowledge.