Good point to make Syl. The value of any vintage or Antique item is a very dynamic one, True value of a item is what at least two people together are willing to pay. There are several kinds of values. First is the Insurance Value, Top dollar. (what would it cost you to replace the item when something happens. Market value, is close to true value, here look around the net and you will get a ideal of value. Here it is important to know the item as much as possible, History, maker, date and etc. Then you have dealer price. A dealer to operate a business must purchase a item for 50% of the market value, espically if they have a Antique shop. This to cover wages and overhead, not to mention the profit.
The next consideration on value, is how quick you need to sell. If you wish to yield market value for a item, Then consignment shop, Antique mall or online store is what it would take. Quick and little under the market is the world of Ebay, though sometimes you can get lucky a get higher then market value, but usually about 50% to 75% of value. Then you have take it to a dealer and he will do his best to get it cheap to cover his cost and college fund for the Child. Most dealer I have talk with, when estimate for purchase is kind of not offer real info. However when asked they may give you a better ideal of it true worth. Talk with them about the item first and not let them in on the fact you are looking to sale until you have some ideal of value. This is not a bad game, it the world of buying and selling.
If not in a hurry to sale the item, surf around talk with us here and snoop a bit and the market value will become clear. And with Maker, year and etc. You could even get 90% of value on ebay, but the dealer well that is just how it works always consider 50% is the top they can offer.