A Michigan family was cleaning out an old farmhouse and accidentally stumbled across a long-sought after collection of football cards worth thousands of dollars and considered perhaps the rarest such collection in history. The set is highlighted by an "anonymous" card of former Harvard football player John Dunlop, which was first issued in 1894.

The Dunlop card alone is reportedly worth $10,000, according to Lou Brown, president of Legends Sports and Games. "If it was in the right condition, it could be worth up to $60,000," Brown told Yahoo! News in a phone interview.

"We get a lot of calls from a lot of people saying they've got something, and usually it's not what you expect," Brown tells local affiliate Fox11. But Brown says this set is something different entierly. "It's the 'Holy Grail' of football cards," he tells Fox11.

The Dunlop card, created by the Mayo Tobocco Works of Richmond Virginia, is called "anonymous" because it did not actually feature Dunlop's name. The entire set is considered the rarest football set in history.

Brown tells Yahoo! News that the Dunlop card is being put up for sale by the Robert Edward Auctions this May.

There are only 10 Dunlop cards known to still exist, with some valued as high as $18,000. The entire collection is the first ever to dedicated to football players. And since there was no NFL at the time, the set focused entirely on the nation's 35 best Ivy League college players, according to the site FootballCardShop.com.

You can view some of the other rare cards from the collection here.

The family also discovered several rare boxing cards, first issued by the same tobacco company in 1890. "I was hoping there might be some baseball cards in there too," Brown, who has been trading cards professionally for over 35 years, told Yahoo! News. "But I'm pretty excited with what they did find."

Brown says the set will be evaluated for their estimated total worth, then either auctioned or purchased by Brown's store directly.

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The trading card industry has faced many obstacles in recent years, with competition for fans' dollars and attention going to video games and other non-sports trading card collectible games, like Pokemon and Magic the Gathering. Steven Merriam of the Bleacher Report has written about the decline of the sports trading card industry, placing at least some of the blame on the industry itself for targeting children instead of adults. "Cards are predominantly bought by adults anyway, so I believe they should go back to the way they used to be," Merriam wrote.

"It's a whole different deal now," Brown tells Yahoo! about the ever-changing industry. His own collectible shop, first opened in 1988, now has an entire room dedicated to gaming and other non-sport collectibles.

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