Photo courtesy of Lyn Knight Auctions

The first week in May marks National Postcard Week. Early in the 20th century, postcard enthusiasts sent "Postcard Day" postcards on May 1st, but the modern observance began in 1984 as a way to promote postcard collecting and sending. Don't toss that shoebox of old postcards you found in grandma's attic—they tell stories about the past and can be valuable.

The U.S. first allowed postcards to be mailed in 1872 for 1 cent. The postage stamp can date vintage postcards—it went up 24 times from 1872 to 2013. Postcards mailed before August 1958 cost up to 2 cents. In January 2013, the rate jumped up to 33 cents. Another dating tip: the reverse side of a postcard typically showed the address, postmark and postage until 1907, when a space was created for a message.

Most picture postcards collected today date after 1910. They sell from 5 cents to hundreds of dollars. Price is determined by the picture on the card, condition, rarity or sometimes the message on the card. Even the stamp or postmark can add value. Cards with pictures of street scenes with stores, amusement parks, advertising, people, or special events are very collectible. Some collectors look for postcards with pictures of early autos, trains, or airplanes. Comic—even naughty—postcards had their heyday around World War I. Postcards with reproduced artwork by particular artists or publishers can be valuable.

This vintage postcard sold for $15 at Lyn Knight Auctions in Kansas.

Note: Look for the June issue of Kovels on Antiques and Collectibles newsletter—it features a sale report on early 1990s postcards with cat illustrations by British artist, Louis Wain.

Find prices of postcards in the free price guide at