As an FFL, I often have customers ask me to ship firearms for them. It is usually cheaper than them doing it themselves, and many dealers will not receive firearms shipped to them from private parties. I receive the firearm onto my books (owner, address, firearm type, manufacturer, caliber) and transfer it to a receiving FFL from my FFL firearms log books. It is a traceable transfer.
It is my understanding (and that of the BATFE agent I asked) that you cannot just drop guns off at your local FFL already boxed and ready to go. Your FFL is taking possession of the firearm, so he needs to log it into his books. I have to do this by law whenever a firearm is left at my shop for more than 24 hours, as I am certifying that the source, serial number and description of the firearm is accurate.
As far as how to sell the firearms, talk to your local FFL and either sell them to the FFL or put them on consignment. Many dealers will put a consigned gun on one of the online firearm auction sites as a matter of fact.