Asking what baseball cards worth is, as a phrase, usually means one of two things: what would somebody pay me, or what is this thing notionally worth. Only the first has an answer, and it is found in completed sales rather than in a guide. This page is the short route from a card in your hand to a number a buyer would actually meet.
The number that holds is a transaction
A price only holds if somebody paid it. That rules out asking prices, which can sit unmet for years, and it rules out guide figures, which are an editor's estimate rather than a record. Completed sales are the only category of number in this hobby that cannot be wished into existence, and every method worth following starts there.
Five, and not the first five you see
Take five sales of the same card, in the same condition, in the same form, from the last few months. The filter matters more than the count: a parallel mistaken for a base card, or a graded sale counted as a raw one, will wreck the answer more thoroughly than using four sales instead of five ever could.
Throw away the best and the worst
Then ignore the highest and the lowest and average what is left. Two bidders who both wanted the same card at the same moment produce a price nobody else will pay, and a misspelled listing that almost nobody found produces one you should not accept. The middle three are the market; the outer two are the accidents around it.
And then take the costs off
If you plan to sell, the figure you keep is not the figure the card sold for. The marketplace takes a percentage and the postage comes out of your side, and the checker on this site subtracts both if you tell it what they are. On low-value cards the two together can be most of the price, which is worth knowing before you list.
Questions people ask about baseball cards worth
Why not just use the highest sale I can find?
Because you would be pricing your card against the single most motivated pair of buyers of the last few months. If you list at that number the card usually sits, and sitting costs you the fee on a relisted item and the time.
Do I include shipping in the sold price?
Read it consistently. Some sales show the item price with postage separate and some bundle it, and mixing the two across your five sales introduces a difference that has nothing to do with the card.
What if the card has never sold?
Then there is no price, and the honest position is that the card is unpriced rather than valuable. It may well sell eventually; the number it sells at is not knowable in advance from nothing.
Does a certificate of authenticity add value?
Only if a buyer trusts whoever issued it. Manufacturer-signed cards carry their own statement on the card; third-party certificates vary enormously in what a buyer will accept, and the sold prices are where that shows up.