That is a stencil horn made by either Buescher or Martin for sale through the Lyon & Healy company's chain of music stores.  The first thing you need to do is have the horn looked at to get an idea of the cost to get it into top playing condition.  If it needs much work, it could exceed the value of the horn.  Also, find out if your horn was made by Buescher or Martin.  If it has toneholes consisting of heavy brass rings soldered onto the body it is a Martin.  Check and see if you can find any closing prices for comparable horns on eBay.  Adjust your price expectations according to the condition of the horn.  Stencil horns generally don't have a lot of value, but Martin stencils are more highly regarded than other stencils and fetch better prices.  If it is a Martin, advertise it as a Lyon & Healy Martin stencil.