I just did some recent (like, yesterday) research on this, and UPS will ship a long gun for you however you like, private person to FFL, even with cheaper multi day services, but will only take a pistol as a next day, same with Fedex. I have sent rifles UPS, I think it was 20 bucks or so, and a 3 day delivery time, but the pistol shipping stuff is a new animal to me. USPS wants nothing to do with a non-ffl shipping a pistol anywhere.
I rarely sell guns, only to buy more stuff. And usually, if I'm selling one, there's a friend that has expressed interest in it prior to my deciding to sell it, making it easy :)