See Coins; Stamps; Baseball Cards; Personal Property: There are now price guides for everything in the range of "collectibles", from Depression Glass to Pinball machines, from Rosewood Pottery to Pez Dispensers. Everyone who "deals" in the arena of a particular "collectible" knows the guide values but there may be vagaries (most notably) based upon the condition of the object. See Baseball Cards. 

Three notes. First, all genres have subtleties. For example, there is one shade of Fiesta Dining Ware (a green variation) that commands a premium well above others. Second, the "condition" of the item is key and can make a huge difference in the value of an item. Condition cannot be underrated in the valuation of a collectible. Third, price guide values have limitations since the items in question may be subject to a lack of marketability. Dealers generally pay half of the price guide value, unless it is an item that the individual dealer knows he or she can move it with speed.

The Antiques Road Show has fostered the impression that any piece of junk that is lying around one’s attic is or may be a valuable piece of Americana. Maybe. But it is more likely it is just junk that might draw a dime at your next yard sale.

The rise of e-bay as a nationwide clearing house allows a decent place to get a handle on some collectibles.