Features
All baseball cards can be graded based on their physical appearance. Grading card companies charge a fee to grade each card, though they do offer discounts on bulk submissions by collectors. They also charge shipping and insurance fees if you're mailing the cards for grading.
"Gem mint" is the highest grade for a baseball card. It's very hard to receive this grade when you submit a card. A card in gem mint condition is flawless in terms of the sharpness of corners, centering on the front and back, color registration, clarity and being free of any stains or surface imperfections. There is a very small difference--often not visible to the naked eye--between a gem mint and a mint card. A card in mint condition is basically flawless. The higher the grade, the higher the value of that card. After mint, grades follow in this order: near mint to mint (nm-mt); near mint (nm); excellent to mint (ex-mt) excellent (ex), very good (vg); fair, and poor.
Some baseball cards have special additions directly on them. For example, you can find baseball cards with a piece of fabric swatch from a jersey worn by the player on the baseball card. Some cards are autographed. Some include pieces of game-used equipment, such as bats. Cards such as these can be worth more than a normal baseball card, but demand sets the price. If the player featured on the card isn't that popular or the card isn't that rare, the demand suffers--as does the value.
Types
There are numerous types of baseball cards. You can find a basic pack of baseball cards at a grocery store. The cards found in these packs will normally be less than a quarter apiece. More expensive baseball card packs will contain special insert cards. Rookie cards are usually the most sought-after cards in packs. You will obviously have to hold onto these rookie cards for several years, however, in order to learn whether or not the player becomes a star and whether the card's value appreciates. If the card was mass produced or is easily attainable in a high-grade condition, this lack of scarcity will keep the price down. This is why cards from before 1980 generally hold their value; they weren't produced in such great quantity, and because kids collecting then weren't collecting with future value in mind, many of these cards were either not taken care of or were destroyed.
Time Frame
Just as with stocks, the prices of baseball cards are often determined by events taking place in the sport. Mark McGwire cards became very expensive during and after his pursuit of the single-season home run record. Now that his record has been broken, though, the prices for McGwire merchandise dropped considerably. Baseball cards of older players increase in price once that player dies. Manufacturers will also create special sets in memory of these players, which can grow in price as time goes on.
Considerations
There are two ways to get a good idea of the worth of your baseball cards: from dealers at shows or in stores, or in pricing guides. Current and classic baseball cards are listed in pricing guides, such as ones made by Beckett. These guides will tell you the market value of your cards, but they're only a guide and aren't always exact representations of what the cards will fetch at shows or online. A certified dealer will be able to grade your baseball card by the system listed in the "Features" section. Only after a card has been officially graded will you have a solid idea of the price of that card.
Expert Insight
Vintage cards--those produced before 1980--generally hold their value much better than newer cards, whose popularity and value are often fleeting.
If you buy an unopened set of cards from a particular year, leave it that way. Unopened sets are worth more money.
Visit a card dealer at least once a week. Collectors obtain new baseball cards on almost a daily basis. One of these new acquisitions may be the card you need in order to finish a particular set. When you become friendly with the dealer, he may even contact you when he gets new cards that are worth money.