Though I've never done it before myself, as has been stated, you must tell the buyer that you are not an ffl, so he can check if his ffl will receive the type of firearm you will be sending, and will receive from someone who isn't licensed. You need a signed copy of the receiving dealer's ffl. The receiving ffl needs to know who the firearm came from, so they can log it into their bound book, so a copy of your driver's license is smart to send along. You must have the buyer find out whether the ffl they choose, will receive the type of firearm you are sending. Around here, there are some ffls that won't do transfers at all, there are some that will only transfer "antiques", and there are some that will only transfer hunting type firearms. It's there license, and I guess they can do as they wish, as much as I don't like it.
You should also check the atf website with the ffl number, to make sure the receiving ffl has a license that is current. Check here:
https://www.atfonline.gov/fflezcheck/
Make sure you put in the ad or auction that the buyer must know their own state laws, regarding whether the firearm is ok in that state. As has been stated CA has some current firearms, that are not ok to be sent, and there are some states where current or "postban" "assault rifles" are not ok to be sent. If you send accessories, like normal capacity magazines with a semi automatic, make sure they are legal to send with the gun to the state you are sending it to.
Make sure you have the buyer's name and phone number easily available for the receiving dealer to see, so they know who the firearm is for.
I would make sure ahead of time what the approximate cost of shipping with insurance is, and also require a signature from the receiving ffls end.
Remember, only a dealer may send a handgun through the USPS, and I believe that UPS and FedEx require next day air shipping of handguns.
Realistically, you will get hosed if you try to sell or trade in a gun at your local dealer, because they have to make money on the gun, and the internet gives you the best shot at getting a decent price.