Baseball cards deliver poor returns on investment
People who bought baseball cards from the 1980s were less fortunate. The Wall Street Journal (subscription required) reports that "Two things worked against these investments. The first is that trading cards were too popular in the 1980s. Card companies printed a lot of them and young collectors kept them in pristine condition. Thus they have no relative scarcity of the kind that makes famous ones like Mickey Mantle's 1952 Topps card so valuable."
It's a classic case of reflexivity: Baseball cards became so collectible that they didn't have any value. That rule applies to nearly every product that is marketed, and it's why collectible plates offered in infomercials and Parade magazine advertisements have generally made such abysmal investments.
It's also why you should absolutely not under any circumstances even think about buying this absolutely moronic Barack Obama The Movement For Change Collectible Express Train Collection -- available for just $69.95 per issue, plus $8.99 for shipping of course. Anything that is being marketed by its manufacturer as a collectible is a lot more likely to make the seller rich than the buyer.
Given the costs associated with storing and insuring collectibles, they're almost certain to be a less lucrative investment over the long-term than traditional asset classes like stocks, bonds and real estate. Of course if you derive enjoyment from your thimble collection, that's a return on investment unto itself.