The best way I have found to get a real value of a used gun is to use gunbroker:
1) Create an account, if you don't already have one.
2) Select "Advanced Search"
3) Up at the top of the search box, select the option for searching "Completed Auctions"
4) Enter your firearm in the search box
5) Find the drop-down menu for how you want results sorted, and sort from most bids to fewest.
6) Hit search
By doing it this way, you are looking at what people are paying for your gun, not what people are asking for your gun (Don't forget to take shipping cost and FFL transfer into account). You can browse through the listings to find one that is in similar condition based on the pictures and description provided.