About 12 years ago, Jon Frank purchased a duck decoy at public auction for $15,000. That same day, he resold it privately for $17,000.
"It's a blue-chip commodity, as far as I can see," said Frank, who now works for family-owned auction house Frank & Frank Sporting Collectibles in Howell, N.J. "The better the decoy is, the more it will increase in value over the time it's in the person's collection."
Waterfowl decoys are hotly sought-after collectors' items, as are many other treasures that might not initially register as potentially valuable collectibles: car hood ornaments, watches and clocks, antique fans, Pez dispensers and even condom tins.
But, as with most investments, the value depends on marketplace demand.
"Pez was doing well, but not as much anymore," said Terry Kovel, co-author of "Kovels' Antiques and Collectibles Price Guide." "If there's too many of anything, they don't go up in value."
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Dale Pike, of Pittsfield, Mass., has amassed more than 1,000 Pez dispensers over the last couple of decades. His collection started when he and a friend were driving across the country and offered Pez candies to toll booth operators instead of money.
"No one took us up on it," Pike said. "But no one called the police on us, either."
Pike said that if he were to sell his collection at a fire-sale price, he might get $5,000 for it—so obviously the collection is not a nest egg.
"You start off collecting for a dollar or so apiece; then you branch out to $5 or $20 for one; then you get serious and pay a few hundred dollars for one," he said, adding, "You really do it for the love of it."
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A recent eBay listing of a single Pez dispenser was $499.95. It was for a vintage bunny dispenser—pink body, white head.
Meanwhile, decoys have continued to rise in value since the 1960s.
Harry T. Williams, of Worton, Md., purchased three dozen duck decoys for 50 cents apiece about four decades ago. He eventually resold all but eight of them. Those eight became the start of his collection.
While Williams realizes the investment potential of decoys, he does not make purchases for that reason. "I look for what I like," he said, adding that he focuses on local carvers—that is, Maryland-based artists.
Frank, of Howell, N.J., echoed the story. "I hunted and fished all of my life, and I loved the form of a decoy," he said. "I ultimately took one decoy out of my hunting rig, and that got me started with collecting."
A duck decoy sold in January at Sotheby's in New York for $767,000. It was a male eider created around 1900 by an unknown carver.
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"It's like anything else; you really have got to know what you're doing or get help from someone who knows what they're doing," said Nancy Druckman, head of American Folk Art for Sotheby's.
"There are infinite ways you can go wrong," she said, explaining that decoys can be fakes, the heads could have been replaced, or an uneducated buyer might overpay for an item.
And the fan collectors? Yes, they are fans of fans.
Dick Boswell, of Lincoln, Neb., is among them. He has a collection that, he said, dates to when he was a toddler.
The baby book Boswell's mother made for him shows that his favorite toy when he was 2 years old was an inoperable fan. He carried it from room to room simply to spin its blades.
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He now has a room in his house devoted to his fan collection, where the items are on shelves and in display cases.
Most antique fans, Boswell said, go for several hundred dollars, although he once saw an eBay-listed fan fetch $13,200.
Boswell is treasurer for the Antique Fan Collectors Association, whose members will meet in July in Indianapolis for what they call Fan Fair.
Meanwhile, even condom tins carry value. Kovel explained that collectors are drawn to the illustrations on the tins, along with the fact that they are no longer made.
Then there are people who collect anything car related, including spark plugs, chauffeur licenses and hats, and the flower holders that some early limos featured.
"Hood ornaments can go for a fortune," Kovel said. Indeed, one was listed last month on eBay for $199,999.
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There's even a market for collectible sneakers. Joe Diorio, owner of Brooklyn, N.Y.-based SoleXChange, saw a pair of Nike Air Mags at his business go for $6,900 earlier this year.
"The market for sneakers is mostly young adults," said Diorio, whose business basically is a place for people to buy and sell sneakers. "It's booming, and mainstream America is catching on to what we love to do."
A limited number of Nike Air Mags, based on shoes worn by Michael J. Fox in the "Back to the Future" movie trilogy, were released in 2011.
Diorio's company holds events in various cities, where attendees can buy and sell. On May 4 SoleXChange will be in Philadelphia at Temple University; on May 31 the company will hold an event in Freehold, N.J.
However, starting collections for their investment value is a crapshoot.
"In general, these are considered speculative types of investments," said Russ Robertson, a certified financial planner with WealthCrest Financial Services. "You can get great returns, but it comes with high risk, unless you really know what you're doing."