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.