Ageless rock band Chicago references the cardboard collectibles in their 1975 single "Old Days".
According to the Wikipedia page the song was written by James Pankow for the band and sung by Peter Cetera. Quite interestingly, Cetera despised singing it because of the reference to the Howdy Doody show which he hated.
Baseball cards at 0:45
I've heard the song many times, and but for some reason it really jumped out at me lately and made me wonder if there were more.
Searching for others led to the discovery of a song titled "Baseball Cards" by the Wavves a San Diego based surf-noise rock band. Strangely the song makes no reference to cards or baseball at all.
Further the post-rock-emo outfit The Appleseed Cast recorded the song "Marigold and Patchwork" in 1998 and it contains a reference to hearing baseball cards roaring in the spokes of a bicycle, (2:16, listen here).
The 2002 single "Capturing Moods" by indie rock band Rilo Kiley refers to selling baseball cards for rent (2:28, concert video here). Personally I'd rather be homeless.
"If I Only Had a Heart" a catchy 2001 single by the power-pop rock band Ozma mentions "...selling it all at the baseball card convention". (1:48, listen here).
There are more...surprisingly a lot more. Here is just one of lists.
May it be on the radio, your mp3 player, your 45's, or whatever your listening preference, listen carefully, maybe you'll run across a song with baseball cards somewhere in the lyrics.
If you have a favorite, feel free to drop it in the comments.