How to value baseball cards is a four-step method and the first step does most of the work. Identify exactly which card you have, find sales rather than listings, take five of them in your condition, and reduce them to one figure by throwing away the extremes. Everything else is detail around those four moves.
Step one: identify the exact card
Year, manufacturer, set, card number, player, and the parallel if there is one. All of it is printed on the card, usually on the back. This step is where most bad valuations come from: a base card priced against a numbered parallel, or one year's set against another's, produces a figure that has nothing to do with what you are holding.
Step two: find completed sales
Search the marketplace where the card actually trades and switch to the sold or completed filter. Active listings are asking prices and can be any number at all. If the sold results look surprisingly low against what you had seen advertised, the filter is almost always the explanation rather than the market.
Step three: match the condition
Compare raw to raw and graded to graded at the same grade. Within raw, be honest about corners and surface: most people rate their own cards a grade or two high, and the price difference between a sharp card and an average one is large on anything common.
Step four: reduce five sales to one number
Drop the highest and the lowest and average the middle three. The card value checker on this site does it, prints the straight mean beside it so the outlier is visible, and shows the spread so you can judge whether your five sales agree. If they do not, the answer is better comparables, not more averaging.
Questions people ask about how to value baseball cards
How long does this take per card?
About a minute once you are used to it, and most of that is the search. It is worth doing properly on a short list rather than roughly on a whole box.
Should I use a price guide at all?
As orientation across many cards at once it has a use. For the single card you are about to sell, a guide figure is somebody's estimate and completed sales are what a buyer paid.
What if my card is in a slab?
Then search sales of that card at that grade in that holder. Graded sales cluster more tightly than raw ones, so you often need fewer of them to be confident.
Do I include the buyer's postage in the price?
Be consistent across your five sales. Mixing bundled and separate postage introduces a difference that has nothing to do with the card.