http://www.armslist.com.
You still really need to know what you are buying is worth, because some people like to try to sell their used gun for more than a new gun. Gun dealers can also list guns on Armslist. I found a really low margin gun dealer that is local based upon their listings in Armslist, e.g. $99 AR15 lower receivers.
You will also be surprised at the number of people that list their weapons for sale, but will not return answers to any of your inquires. Having said that you will also meet some really good people and good deals on some esoteric weapons.
just like Craigslist indeed. to be more like CL,they should first make an offer of trading for a lawnmower they have or offer you half your asking price without even first seeing it or asking a preceding question.