In an article on baseball card collecting Slate.com's Dave Jamieson gives an explanation of how the value of baseball cards have plummeted over the years. I think there are some explanations in this article of how other collectibles have also lost their values to collectors.
I also think these kinds of things are cyclic and the lower the value of Baseball cards becomes the more valuable they may become for future collectors.
Requiem for a Rookie Card: How Baseball Cards Lost Their Luster
Baseball, collecting, selling