Age matters. Many of the sports cards that can be found floating around yard sales and the like are from the late 1980s and 1990s, when the card companies were cranking out tons of cards. Today, most cards from that era aren't worth much.
What you want to look for are cards that are older (especially if they are pre-1980) or current (say, five years old or less). To check, look for a year somewhere on the card fronts or go to the card backs and look for a copyright date. Even common cards from some vintage sets can bring some money, so be sure to keep an eye out for older cards.