IN the last decade, the hobby of collecting and trading baseball cards turned professional. Fueled by nostalgia, the aging of baby-boomers, and the bottom line of investment appreciation, the intangible memories of childhood coalesced into cash sales.Not only cards but also a wide-ranging assortment of sports memorabilia - autographs, as well as gloves, bats, and uniforms worn by star players - saw their nostalgic worth swapped for dollars. In 1990, according to collectors and antiques industry sources, $1.5 billion changed hands over such items. The rise in prices is likely to continue. "Baseball memorabilia is perhaps the widest collecting area in America," says Karen Keane, managing director of Skinner Inc., an auction house with branches in Boston and Bolton, Mass. The 1980s ushered in a "new collecting market. There's a grace and elegance to baseball," she says, "a slowness coupled to speed and action. It stops time, in a way." And today, there's not a player on the Boston Red Sox that isn't a lot younger than most baby-boomers, says Ms. Keane. "Baby-boomers have reached a point in their lives and careers where they have the money and time" to do some serious collecting, says Steve Ellingboe, publisher of Sports Collector's Digest. With 10 years' experience writing about sports collecting, he emphasizes that the main factor is still nostalgia, not money. Last March, Sotheby's New York office held its first specialty auction of sports memorabilia in the United States. The premier item was a multicolor baseball card, originally issued by tobacco producers in 1909 and 1910, of Honus Wagner, a famous Pittsburgh Pirates shortstop. Wagner did not smoke and strongly objected to his name being used to promote the habit. Very few of his cards were printed and fewer remain. The pre-sale price was expected to top $100,000. The card sold for an incredible $451,000, sending hobbyists scrambling to their attics in hopes another copy could be found. Baseball's dominance of sports memorabilia rests on two pillars: "There exist a lot of affordable things in baseball, and it spans generations from kids to grandparents," says Keane. Former schoolteacher Gary Gilbert, owner of Gilbert Sports Nostalgia store in Needham, Mass., says, "I collected and gave away baseball cards. One day I noticed the kids talking behind my back." It dawned on Mr. Gilbert that they were perplexed, even humored, by his giving away valuable stuff. The experience forced him to decide between what "I sell and what I keep for my hobby," he says. For a long time there was only a market for baseball cards and a few other items associated with baseball, according to Gilbert. Now basketball, ice hockey, and football items are being traded, he says. In 1985, while collecting memorabilia on Boston Celtic Larry Bird, Gilbert bought a Michael Jordan fluorescent sticker for 50 cents. Just three months ago, at the height of the National Basketball Association playoffs, a sticker identical to the one he owns sold for $2,000. The manufacturers of baseball cards will not say how many of each card they print, says Mr. Ellingboe. Ellingboe offers the educated guess that in 1960 there were 100,000 cards printed per player. In 1975 there were 250,000 per player, and in 1990, 5 million. "In the year 2020 there won't be the shortage of the 1990 cards there are now for the 1960 cards," he says. Will the bubble burst? The long-term effects of placing a sticker price on all sorts of sports stuff is still too recent to measure, says Keane. The market determines the price. Among the items her firm sold at auction last week were eight 1889 black-and-white baseball cards of the Boston Baseball Club for $3,000, and a turn-of-the-century series of 140 cards for $4,950. Steve and Leslie Rotman, husband-and-wife collectors, realized sports collecting had changed when they found themselves applying a "reverse pyramid" strategy to acquiring 19th-century sports cards, photographs, and advertising graphics. 'PEOPLE start with something current," says Mr. Rotman, such as baseball cards. Eventually, an increasing number of collectors will want to go deeper, to "go back to a period when few collectibles are around," he says. A lot of contemporary sports items can be collected today. There is not much collectible between 1840 and 1900. If the Rotmans' strategy works, the few people who secure older memorabilia today will be at the apex of the collecting pyramid tomorrow. The weight of all the others who want to collect will drive prices up. No one need fear that money-conscious players will turn the game over to investors, says William Guilfoile of the Baseball Hall of Fame in Cooperstown, N.Y. "Most players feel honored to send us a bat or ball they have set a record with," he says. Even though a number of the big-name players only sign autographs for a fee at trading card and memorabilia shows, the overwhelming majority of players sign almost anything a fan puts in front of them at the ballpark, say hobby purists. But Gilbert says the United States may be entering an era in which society's relationship with sports is changing in unanticipated ways. One of the more troubling aspects of the recent investment craze revolves around what values "we as adults are communicating to youngsters when society places such a high dollar value on something that formerly was theirs," says Gilbert. "When it becomes too investment-focused, we've taken something away from kids," says Gilbert. He likens the current adult obsession with dollar worth to parents who go to a Little League baseball game and vociferously argue with each other or the umpire over a call, or a Little League coach who plays only nine starters and benches everyone else in order to win at all costs. Gilbert has formed a club for kids, "Gilbert's Guerrillas," where he tries to get around the high price of some of the new, top-of-the-line baseball cards through group purchases. Such cards are sold exclusively in lots of 1,000. They are made of high-quality Kodak paper, with pictures on both sides and anti-counterfeit hologram imprints. "I emphasize this as a hobby, not just an investment," he says.