Blue Sharpie would most likely be the most preferred, or a color that will make the signature stand out the most depending on the contrast to the color of the card signed. The better the signature displays the more valuable. A ball point pen might adhere to the card better, but will leave an indent on the card and will not stand out as well. Either way what ever type of pen or sharpie used be careful, and allow the ink to dry fully before handling or placing in a protective cover. If not properly dried the ink will smear. With Sharpie even after the ink is properly dried, and placed in a protective cover, over time the ink could still "sweat" onto the plastic, and fade the signature. You might want to consider having another item signed or a Baseball card that does not have a glossy finish. A high valued collectible baseball card will be devalued by a signature. The glossy finish on the card is also not the most ideal surface for a signature. Although there is a small community of collectors that collect signed baseball cards, Baseballs are the most preferred items for a signature, followed by photographs. A signed baseball card will be worth less than a signed baseball by the same player. For more information on signed items read my newsletter "How Much Is my Autographed Baseball?" There might be some information in it, that you might find useful. I will leave the link below.
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