Auction houses have begun looking for ways to attract younger buyers without alienating the boomers who drive their sales. Inspired by the youthful fashion craze for vintage concert T-shirts, Christie's held its first sale of them in November. A Yardbirds T-shirt worn by rock journalist Greg Shaw to the Monterey International Pop Music Festival, in 1967, sold for $3,000, while a pretty ordinary Led Zeppelin T-shirt went for $1,625. "Younger buyers might not even listen to Led Zeppelin," Lipman says, "but they want the T-shirts because they're cool."

Generation X is smaller in numbers but growing in spending power. Its top earners are Web 2.0 entrepreneurs like the founders of Google and YouTube, newly minted law partners, and corporate leaders who grew up in a computerized world. As this group begins to buy back its youth, new trends are surfacing. Skateboarding culture has produced its own niche collecting cult. Especially coveted are mint-condition "decks" from the '80s, when skateboard art got wildly creative along with the sport itself. An original 1984 Phantom recently sold for $7,250.

Where baby boomers are more likely to drop $6,000 on a '50s-era brunette Barbie, Gen X is intrigued by the boys' toys of the '70s and '80s. Star Wars action figures still in their cases bring impressive prices, as do Hasbro's Transformer series. Doll-size robots like Targetmaster Scourge, purchased for a few bucks in 1987, now sell on eBay for $5,000.

It's not too late to get in on Gen-X collecting trends. Unlike boomers, this group is relatively new to frivolous spending. "You have to look at what has really gotten people excited," says Lipman, who is 33. "Unlike kids from the '50s and '60s who would go outside and play ball and pretend they were Mickey Mantle, we would stay home and play Super Mario Brothers. Now people are trying to get their hands on the systems and games. It wouldn't surprise me to see computer memorabilia and even computers themselves take off. Rare ones like the original Macs already go for lots of money."

What will appeal to the millennial generation is anyone's guess, since its members have yet to accrue much disposable income, let alone spend it. But limited-edition items related to Harry Potter are likely to rise in value among the children who waited in line for J. K. Rowling's latest releases. So are trading-card games like Yu-Gi-Oh!, avidly collected by kids in the late '90s. Rare prize cards such as Doomcaliber Knight can fetch a couple thousand dollars on eBay, and the category in general—and related videogames—are likely to appreciate as those kids age.

Of course, not every collection is based on reliving youth. How else can one explain the continued fascination with Marilyn Monroe? Demand for silent-era mementos has faded, and Hollywood classics are losing ground as collectors who came of age in movie palaces give way to those who watch films on iPods. Yet Marilyn endures. "For the last six years, Marilyn Monroe has been our biggest seller at auction," says Margaret Barrett, director of entertainment memorabilia at auction house Bonhams & Butterfields. "She died in 1962, so she's pre–baby boom. Yet she appeals to collectors in their twenties and seventies—male and female. Every generation seems to discover her."