Evaluating collectibles is very much a matter of judgement, but there are plenty of price guides out there which define the parameters which are to be used. These are the things that most appraisers take into account when placing a value on a collectible. Collectibles are generally evaluated on the following bases:
Collectibles which are in quite good condition (without any signs of wear and tear) generally fetch a more premium price when compared to something which is in a bad condition. Collectible which look like new are known as 'mint'. Some collectors consider the condition as one of the most important factors when evaluating a collectibles
The degree to which a collectible is rare can also be a big factor in deciding on its value. A good example would be the first Ponytail #1 Barbie w/brunette hair, which is much harder to find than the usual blond ones and, therefore, commands a higher price. If there is a wide variety of the same thing, then the least findable ones are more precious.
Age is a very good way of finding out the value of a collectible, because there aren't a lot of collectible which have survived the test of time. However, this may not be the case for all collectibles. Some of the new action figures are really popular to such an extent that such action figures from, say, the 1970s, may not be so interesting. In order to find out the item's rarity, do a bit of research or get the help of an expert.
The packaging that comes with the collectible can really add to the collectible itself (in value, too) and can give a good idea of how old the collectible is, and who made it. Some of these action figures sell for a higher price if they have the original box, rather than one which doesn't. Since people who buy these action figures tend to throw the original packaging away, it gives the collectible a bigger sense of rarity.
Some collectors tend to have more interest in collectibles that have come from their own place, so the value of such items would fetch a good price if sold there. To give you an example, a Pennsylvania Dutch furniture will draw a bigger price in the East Coast than in, say, California. Likewise, California pottery will draw less interest in Massachusetts. Books written by authors from the region will, too, draw more interest.