Age
Although generally speaking the older a coin is then the more it is worth. However, but this may not be the case.

Condition
Unfortunately a coin which is badly corroded and you are unable to tell what it is will not actually be worth much. But in more recent coins the fine degrees of condition can mean the difference between coins being valuable or it is just a curiosity.

Metal Content
As we all know the coins that contain high amounts of gold or silver are more valuable than those that contain copper, nickel or zinc. But there is always an exception to the rules and in some cases there may be a very rare copper coin that can be worth lot more than some more common silver coins.

Rarity
This is probably the single most important trait when valuing a rare coin. Sometimes coins that contain mistakes are often the most valued. As mistakes do not happen very often and are often quickly caught by the mint if such a coin does make it into circulation with a mistake then it can be worth a lot.

Country
Unfortunately in America there is not much of a market for world money. In fact they are more of a curiosity rather than a commodity to many coin collectors in the US. But if you are not collecting coins for their monetary value then collecting world coins can be a lot of fun.

Personal Preference
This should not be ignored. Although one coin may be priceless to one person it may mean nothing to someone else. There are many collectors who collect particular coins because of their sentimental value (it may have been a gift from either a friend or a relative).

Market Value
If there happens to be a high demand for a coin at any one time and if you happen to be selling one then you are more than likely going to get a good price for it. But just like fashions coins go in and out of style. So if you have a coin that is out of favor at the moment hold on to it until it comes back into fashion. The coins that tend to fluctuate the most are those made with precious metals.

But when looking at a rare coin value estimate it is best if you leave this to an expert. Such people are better able to judge the market more accurately and will be able to tell you when is the best time to buy or sell a certain coin.