To determine the value of an old postcard first classify the condition of the card, and then consider the rarity, age and subject matter. To finalize an estimate, look into current market desirably and demand for certain ages or subjects of cards.
Condition is extremely important to consider during an evaluation. A postcard with damage, even if it is antique or rare, does not hold much value. Look for discoloration, tears or foxing on the card. Also determine if the card is intended to have any embellishments, such as glitter or gilding, and if so, how much of the embellishment is still in place.
The age of a postcard is hard to determine at times. Some features that help to age a postcard are the type of card stock it is made from, if it has a divided back, the type of imagery and if that imagery has a white border. A card that is printed on linen texture paper appears woven under inspection and is from 1930 to 1944. An undivided back on a postcard dates it to 1907 or earlier. An image on a postcard that has a white border dates from 1915 to 1930. A postmark is also helpful to determine the age of a card but only as a starting point, as the postmark shows the date that the card was mailed and not the printing date.