If you set up an "education service" across the street on private property with permission to be there, there isn't a damn thing anyone can do about it. A Bluebook and handouts with a list of local gun dealers,and a handout with your state's applicable laws would be a nice touch. A really large sign with some values printed, such as "Smith and Wesson Revolvers: Typical Value: $250 to $500", ".22 Rifles: $80 to $500", and so on would be a uselful addition. Include a teaser at the bottom that tells the value of some examples of the example firearms is much higher if rare, antique, or in high demand.
Private sales to you are dependent on your state's laws, and regardless, the truly uninformed that are in line at the buy back are likely to be uneasy and usually unwilling to sell a firearm to an individual. For all they know, everyone in sight needs a permit just to look at a firearm. But you never know what might appear out of grandpa's sock drawer.