I cannot tell of the lacquer is original from the photos.  This will have a large impact on price, and you should find out if this is the case prior to trying to sell.  

Here is an Ebay auction that you can follow.  This is a worn, but original lacquer version of your horn.  If yours is original lacquer, you should be able to get about $300 - $500 more for your horn if you sell directly to another player.  You will only get about 1/2 of that if you sell it to a retail establishment.  If on the other hand, the horn has been relacquered, then you may only be able to get $100 or so more that the referenced auction.  http://www.ebay.com/itm/Buescher-Aristocrat-Series-III-Alto-Model-140-/222637783610?hash=item33d63efa3a:g:cG0AAOSwRwhZsWHG

Recognize that with 40+ year old pads, your horn will need a pad replacement in the near term, and that would run a buyer about $500.  That is one of the reasons the older vintage horns sell for what they do.  Unless you know someone who will give you a real bargain on a repad, I would leave it as is, since I would not want to risk marring the finish before selling.

Here is another auction that may be closer to your horn, but you will have to wait quite a while to see what it sells for:  
http://www.ebay.com/itm/Vintage-1950-51-Buescher-Aristocrat-Alto-Sax-Model-140-True-Tone-/222659011739?hash=item33d782e49b:g:a84AAOSwSnFZpJhh

Good luck with the sale.