Here is a word of caution for new stamp collectors.

Stamps were usually, not always, produced in sets or in a series. An example is the Lousiana Purchase set of stamps issues in 1904. This attractive set consisted of five stamps in denominations of 1,2,3,5,and 10 cents. Most of the time the higher value stamps in a set are more expensive than the lesser denominations. Beware of people selling a "Set" which is actually what is called a 'short set' of stamps. You could, for example, see this set available with only the 1,2, and 3 cent stamps. This is an issue with both US and foreign stamps.