Condition

• To gauge the condition of a card, buyers look at factors like corner sharpness, photo centering, whether there are bends or creases, and whether the card's edges show signs of wear and tear.
  
  

Grading

• Grading has become the prime method of determining a card's condition and value. Companies including Professional Sports Authenticator look at a card's condition and issue it a grade, which can increase or decrease the value.
  
  

Age

• An older card--especially one from before the 1980s--in good condition can be quite valuable to the right person, because such cards can be hard to find in good condition.
  
  

Types

• Baseball cards are no longer limited to presenting player pictures and statistics: Some of today's cards include player autographs, materials used in the game--like bats and jerseys--and other special features. These cards are typically scarcer, and therefore carry more value with buyers.
  
  

Guides

• Price guides like Beckett and Tuff Stuff have print and online versions that give the approximate value of a card.