I buy and sell on Armslist every now and then. I always do it locally. Many people ask to see ID, or meet at the local police station. Some police dept's here have a "safe room" where exchanges can be made.
The benefit of Armslist is, like Craigslist, you can sell something at a more reasonable price for you, not have to settle for a low price from an FFL that is looking to maximize their resale potential. Just like you would do to get a better price selling your used car. Also, you might find something for sale that no FFL has in stock.
The down side is, so many sellers ask high prices for used stuff. I think many expect to be offered less, so they start high to begin with. Some want to make money on their sale. When you see something on sale that is $50-100 higher then new prices at Academy, you have to wonder what they are thinking.
I think Armslist, and Gunlistings.org, and other sites like them, do a service to gun buyers, I'm glad we have them.