While Rosen is passionate about both baseball cards and robots, he makes a distinction: Cards are his business, and robots are his fun. He started his collection in 1954, when he was eight years old and his parents bought him his first robot. As an adult, he stopped his other collection — of documents signed by presidents and signers of the Declaration of Independence — so he could afford his robot collection.
''A collector usually collects what they had as a child,'' said Rosen, 61. ''Collecting is sort of buying back a piece of your youth.''
There are enthusiasts for almost any collectible, judging by the clubs listed on the Collectors.org Web site, the umbrella for the Association of Collecting Clubs and the National Association of Collectors. The list of 3,566 clubs turns up Beyond the Pond for collectors of frog figurines, Kollectors of Nasty Old Ties (KNOT), Hammered Aluminum Collectors Association and the Lighter-than-Air Society for blimp enthusiasts, not to mention clubs for people who like Christmas ornaments, hunting decoys, salt-and-pepper shakers, thimbles, bricks, bottles, beads, buttons and board games.
Wine is a gaining popularity as a collectible among amateurs and professionals thanks to growing demand leading to sharp price hikes. That has drawn interest from investors who have created funds to treat wine much like stocks. The Fine Wine Fund and Wine Investment Fund, both based in the U.K., buy wines making bets on how different vintages will sell later.
Former investment banker Steve Bachman founded Vinfolio — a San Francisco company that sells wine through its Web site, stores bottles for clients and helps them catalog their cellars — to follow a passion for wine that had already led him to create a personal 7,000-bottle cellar. Bachman's clients sometimes end up as accidental investors.
When Bachman's clients are buying wine for personal consumption, they sometimes add a few cases to their order, with plans to sell them for a profit later.
Bachman generally doesn't encourage wine collecting as an investment strategy, but he concedes that in recent years at least, wine has proven to be a good investment. For example, he bought six bottles of a 2003 Petrus for $670 a piece in July 2004 that have nearly tripled in value, with an average auction price of $1,878.97 today.
''I don't in general advocate people treating wine as an asset class, like securities, partly because that's not what it's intended for,'' Bachman said. ''It's intended to be consumed.''
But the collecting sickness certainly seems to afflict at least some of Bachman's buyers. He reports a near-fanatical loyalty by some Chinese customers for certain vintages from the Bordeaux region of France, popular partly because of its long track record for producing quality wines.
''The press hypes a vintage and everyone wants it,'' Bachman said.
Rising demand, particularly from China, has affected world markets, he said. ''There seems to be almost like a fixation on particular labels people want and pay prices that almost don't make any sense.''
Irrationality seems to creep into most collecting markets.
Rosen said this year there is a set of baseball cards featuring the greatest players of all time. The set of 17,300 cards come in different styles and colors and with different pictures.
''Now, that is really for a sickness,'' Rosen said. ''Try to collect 17,300 of something just because it has a little different picture or little different word — that's where I have to draw the line.''