Collectible football jerseys are valued based on several factors: whether they were worn and by which player, whether they were autographed, and what era they're from. A jersey autographed by a famous player and a worn jersey autographed by that player are two completely different things to collectors. Many players autographed limited numbers of promotional jerseys, which were often sold with a certificate of authenticity.
The other big factor is era, either for the team, or within the individual player's career. Numbers no longer in use by a football team are more valuable, especially those from the team's infancy or a big winning season. For individual players, jerseys from college years are often sought-after.
Interviews & Articles
Going Deep for Football Cards

I collected football cards when I was a kid, back in the late ’60s, early ’70s. Then I set them in the closet for about 15 to 20 y… [more]
Best of the Web (“Hall of Fame”)
Vintage Football Card Gallery

This great database of pro and collegiate football cards from the 1950s and 1960s offers scanned images of cards fo… [read review or visit site]
U. Michigan Football Ticket Museum

Brian Powers' excellent gallery of over a century of Wolverines ticket stubs (1900 to 2005), not to mention a lot o… [read review or visit site]
The Killer Mobile Device for Victorian Women
If These Shirts Could Talk: The Tantalizing Tales Behind Used Clothes
Gloriously Grotesque 19th-Century Pipes
In the Hot Seat: Is Your Antique Windsor a Fake?
Bizarro Beauty Products, from 1889 to Now
Love at First Kite: How Pizza and Pente Led to One Oklahoman's High-Flying Obsession
Pin-Up Queens: Three Female Artists Who Shaped the American Dream Girl
Say Ahhh: An Oral Surgeon's Quest to Reimagine the Garage-Band Guitar
Tokens for Sweethearts, in Times of War
American Picker Dream, Part I: Mike Wolfe On His Love Affair With Bikes

by 

by 