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One-of-One: A card, usually a parallel, that is (literally) a one-of-a-kind item.
The gimmick was introduced in 1997 by Fleer with their "Masterpiece" cards in Flair Showcase Baseball and Basketball. The concept was so successful, that ones-of-one have been a Hobby staple ever since.
In 1997, a California dealer attempted to purchase all three Ken Griffey, Jr. Flair Showcase Masterpieces. '97 Flair Showcase Baseball had a fractured, three-tiered, base set, and there was a one-of-one Masterpiece produced for each tier. He was able to purchase two of the three for $14,000 and $15,750, respectively. The third Masterpiece was found by a teenager in Alaska who was offered an amount rumored to be around $24,000. He refused.
While in retrospect that might have been a mistake -- he probably couldn't get anywhere near that much for it now -- you have to admire the kid. He found the ultimate card of his favorite player, and refused temptation. As a post-script, in the fall of 1999 the two other Griffey Masterpieces were auctioned on eBay. The final bid of $6800 didn't even come close to meeting the seller's reserve price. This just goes to show that there is nothing completely resistant to the power of economics.
Another example of the power of economics, has been the recent trend of certain card manufacturers (read: Donruss-Playoff) to load up their products with multiple ones-of-one for each base set card. The predictable result of which has been the driving down of the aggregate value of such cards. The textbook example of this "proliferation of contrived scarcity" is the 2004 Diamond Kings baseball set.
With a dizzying array of parallels, '04DK was a product unlike any The Hobby had seen before. Among the 79 different parallel sets (some of which had featured articles of game-used equipment, autographs, and combinations thereof), were 20 limited to only one copy. The only distinguishing characteristic between one and another was either the color of a foil stamp, or a matted frame. This has led many a collector to ask: If each individual card in a base set has 20 different one-of-one parallels, can it really be called a "one-of-one?"
The contrived scarcity issue has led some collectors to establish a new guideline between what is, and what isn't a one-of-one. The term "True One-of-One" has been coined to distinguish between a card that is truly limited to only one copy, and products like '04DK which have multiple ones-of-one.