How much are baseball cards worth is a question about individual cards wearing the clothes of a question about a collection. A box has no price. What it has is a few cards that might, and the fastest honest route is to find those, read completed sales for each, and accept that the rest are worth roughly what they cost to post.
Sort before you search
Pull out anything with a serial number, an autograph, a memorabilia patch, or a date before about 1980. Pull out anything that is obviously an insert rather than a base card. That is usually a small pile out of a large box, and it is the pile worth spending time on. The base cards from the late 1980s and early 1990s can go back in the box.
One card at a time, on completed sales
For each card in the small pile, search completed sales rather than active listings, and filter to the exact card in roughly your condition. Five recent sales is the target. Anything with fewer is a thin market, and anything where the five disagree wildly is usually a sign that you have caught several different versions of the card.
Turn the five into one number
The card value checker drops the highest and the lowest and averages the middle three, which is the standard way of describing where a small sample sits without letting one result run the answer. It also prints the mean beside it, so you can see exactly how much the outlier was distorting things before you decide what to do.
Subtract what selling costs
If you are selling rather than curious, put your marketplace's percentage and your postage into the same calculator. On low-value cards those two together can take most of the price, which is the honest reason that selling an ordinary card individually is often not worth the afternoon it takes.
Questions people ask about how much are baseball cards worth
Can I value a whole collection at once?
Only roughly, and only by valuing the best cards and treating the rest as bulk. Collections are sold that way too: a dealer prices the handful that matter and takes the remainder by weight or by the box.
What counts as old enough to be interesting?
Before about 1980 as a rule of thumb, because print runs rose sharply after that and then rose again. It is a starting filter rather than a rule; plenty of pre-1980 cards are common too.
Do I need to get cards graded to sell them?
No, and for most cards grading costs more than it adds. The grading cost and uplift calculators on this site work out whether a particular card clears that hurdle.
Is there tax on selling cards?
In the United States a gain on collectibles is taxed, and the rules differ from ordinary investments. The IRS publishes the topic; it is worth reading before a large sale rather than after.