Ask yourself two questions: "How rare is it?" and "What condition is it in?" The price listed in a stamp catalog gives you some idea of how rare it is. However, the stamp may sell at more or less than the catalog price, depending on its condition. Always try to find stamps in the best possible condition.
The Postal Service Guide to U.S. Stamps (27th Edition) is the new and updated edition of the complete book on U.S. postage stamps.