If you’re buying baseball cards as an investment, switch your focus away from the base cards that you find in all sets. Rather, you need to purchase cards that offer something a little different that could justify a higher price later on.
Rookie cards are a good choice. If you get a rookie card featuring somebody who becomes a star years later, you can usually make a good return. Autograph cards, parallel cards, and any card featuring a game-used relic, such as a piece of a jersey, are also good choices.
Of course, you need to ensure you keep the cards you purchase in good condition to maximize your investment.