I just use a local gun shop. For 20% they will test/inspect/clean, run a check, do the back ground check and if gun is in my name they do paper work to transfer ownership.
They get top dollar and heavy traffic. Web posts. Can sell to someone in any state and the contacts to sell to dealers if you want to sell fast for less.
Usually get,even after 20%, close to what I can get selling thru paper with no work, hassle or risk.