According to hobbyist website The Postcard Guy, there are a number of factors that can affect the value of an antique postcard, including age, condition, topic and rarity. According to Love to Know Antiques, there are many helpful price guides to help collectors price postcards, the most popular of which is "The Postcard Price Guide" by J.L. Mashburn. EBay is also useful to see what prices cards actually sell for.
When attempting to determine the value of antique postcards, many factors must be taken into account. A postcard's age can have a major impact on its value. According to The Postcard Guy, serious collectors seek out cards from 1898 to the 1940s, making these cards generally the most valuable, but cards from the 1950s and on are also appreciating in value. Condition is also important; a card in mint condition will fetch a far higher price that one with creases, tears or other imperfections, which can cut a card's value in half. Additionally, a postcard's topic can significantly influence its price. For example, birthday postcards are extremely common and therefore valued at lower prices, while Halloween postcards are less common and worth more. Finally, rarity is another key factor in determining value. A small run of postcards produced by a smaller publisher will be worth far more than a series that was mass-produced by a major postcard publisher.