I've used Armslist a number of times, in fact the P10c I just bought was through Armslist. It is always a face to face transaction, always cash, and always somewhere safe. I usually ask people to meet me in the parking lot of our local Academy Sports. I have had some goofy responses from people trying to get me to ship. You have no way of knowing if it's a scam or not if you ship. If private transactions are legal in your state, it is a good tool to buy, trade, or sell and get a better price then you might get from an FFL that wants to buy low so he can sell high. It's just like selling your car on Craigslist for a better deal on it.
Good rule of thumb, if it sounds fishy, it probably is. Just don't answer the email. I had a guy try to get me to ship a gun a couple months ago. His user name was some kind of gangster rap name, "Young MC Super" or something like that. I ignored him and he went away.