Do you really want to actually sell your firearms?
Go to gunbroker, look at the COMPLETED sales, look at what ACTUALLY SOLD, and for what price, do not look at what is being ASKED, look at what is being PAID for in completed sales.
OK, now you know what your firearm is probably worth. ...
I think there is something of a value difference between buying online as opposed to buying FTF.
When it comes to buying used guns online, you are buying sight unseen, some sellers post excellent photographs others not so much, some are honest and detailed in their descriptions, others not. I once purchased a rifle online, the ad featured just one photo of rather mediocre quality, but figured I'd take the chance; turns out they touched up a good deal of finish wear with a sharpie, on top of that the trigger would not reset (they didn't mention this in the ad), both of these issues could have easily been detected if buying in person. In most cases, buying used doesn't come with a manufacturers warranty so the importance of being able to inspect is indeed great. Furthermore, buying FTF allows you to go home with your gun that same day as opposed to hassling with forwarding FFL info, waiting on shipping, etc.
I personally would be willing to pay a little more if I could at least have the opportunity to inspect what I am buying first and get to take it with me that same day should I choose to buy it.