I just sold a Ruger 357 revolver last week. Fortunately it went smoothly. A co-worker of mine wanted to buy it. We are in NY but I think it's probably the same all over. If not it will be. This scenario is the same for any firearm here. Our transaction went like this:
1) We both had to go to a gun dealer. (we went to Gander Mtn)
2) The buyer fills out the transfer paperwork provided by Gander.
3) Gander does a background check on the buyer.
4) They take photo copies of both our driver's license and pistol permits.
5) The background check came out good so they take possession of my pistol and give me a paper so I can have it removed from my permit. The buyer pays a small fee and they give the buyer a paper which he has to take to his county of residence to have it added to his permit. (In his county it's done thru the Sheriff's dept.)
6) Once they add it to his permit they give him a coupon to take back to Gander
7) He gives them the coupon then they give him the pistol and walk him out of the store.
Entire transaction could be done the same day but due to his work hours he took two days.
As a seller, I'd hate to advertise on CL or somewhere else. If I did have a prospective buyer I would arrange to meet them at a police station or Sheriff dept. making them aware of what I was doing. I would not want them coming to my house (but that's just me not wanting a prospective buyer or possible criminal knowing where I live). I would ask to see his permit and verify they are who they say. I have a few others to sell but it's harder than ever to do so. I think word of mouth by co-workers and friends would work best for me.
Call your local gun dealer to be sure of current sale transaction laws in your state.