There are many different aspects which come into play when determining the value of collectible post cards. Some of them include the condition of the card, its age, the subject matter, whether it is printed or a photo and the postmark. Postcards are made for thousands of reasons and there also many reasons for collecting them. Some cards are devoted to holidays such as Christmas, Valentine's Day, or Halloween. Some collectors specialize in certain animals such as all alligator cards. Other collect postcards dedicated to specific regions such as Florida postcards, or postcards of the worlds most well known buildings. There are many postcards out there to suit all collecting interests and price ranges from a few cents all the way up to thousands of dollars.
Postcard collectors collect postcards of all ages, but usually the older the card is, the more valuable it is. Postcards made in the United States before 1907 are easily recognized because they had un-divided backs so the only thing that was written on the backs of these cards was an address. After 1907, postcards were divided into two sides on the back so an address and message could both be written. From about 1910 up until the 50's photo postcards were very popular. During the depression linen postcards became popular and were used up until the early 50's. In the late 1940's photo chrome cards made an appearance and are still in use today. These photo chrome cards were made on a wide variety of subjects and are quite popular among collectors today.
Besides age, the condition of the card also has an effect upon its value. Tears, folds, creases, dirt, water damage and postmark bleed through will all decrease the value. When considering a photo postcard, the quality of the photo will affect its value. Such qualities taken into consideration include the contrast, exposure and sharpness of the photo.
Certain types of postcard subject matter seem to be more preferable among collectors and therefore usually make the card more valuable. Railroad and occupational images, Main Street scenes, state views and interior views are usually more expensive to acquire and harder to find. Greeting cards that are popular are Halloween related, full size Santas, 4th of July and Art Nouveau. Of course a lot of it has to do with individual taste and interest, but collectors do like to own some of the rarer cards.
If you are just getting started with postcard collecting, you may want to carry a good postcard value book to guide you when you make your purchases so you will have a good idea as to the value of a particular postcard.