There is Gunbroker.com and gunsamerica.com that are firearms specific. You can do craigs list but not ebay. For that matter you can put it up for sale on this list.
The key to this is what happens after you have found a purchaser. Keep in mind that every state is a bit different and some of the states back east are essentially communist when it comes to gun / people control. However for a locked breach shotgun, very non-threatening, what will essentialy happen is that you will have to find a Federal Firearms Licensing agent / dealer (FFL) and have them ship it to another FFL if in another state or, for that matter, shipped at all.
If in the same state, there are no Federal laws that require anything. For example, here in Colorado, I would hand you the price and you would hand me the gun. That's it.
If you need to ship (think UPS or FedEx) expect to have to pay a dealer at your end. It seems to average $35.00 & shipping. Long guns can go "slow boat" so figure less than $25.00. The buyer will have to pay a like amount to an FFL so in most transactions it is considered a wash and not listed as a cost but factored in the asking price.
The web sites listed above will give you a good idea what similar guns are selling for. I can tell you the market for sporting guns is not doing well. When obama was elected the buying public took their money to black rifles, handguns, and more ammo.
Tell me about it and I can tell you an approx value at least here in the Rocky mtn states. Tell me what you are asking for it. I already have enough of the 3 categories above ;>)