Card sharp is preferred in British English, while card shark is more common in American, Canadian, and Australian English. They share their main definitions—namely, (1) a professional card player, (2) a person who is skilled in card games, and (3) a person who is skilled in cheating at card games. The British card sharp more often implies cheating. Card shark, especially in American English,is often simply a term for someone who spends a lot of time playing cards.
Both terms are sometimes hyphenated, and they’re sometimes condensed into a single compound word—cardsharp or cardshark. But the two-word, unhyphenated forms are more common in edited publications.
Although card shark occasionally appears in British English, card sharp is more common. Here are a few examples:
But, for a day or two at least, he can also wear the half-smile of a card sharp who has outrageously beaten the house. [Independent]
So, displaying the dexterity of a card-sharp, he swiftly removes it from his wrist and discreetly slips it into his pocket. [Scotsman]
The flamboyant card sharp entertained a coterie of friends, including the future George IV, at the property. [Telegraph]
Everywhere else, card shark is the preferred term:
Her character is a lawyer — and a bit of a card shark — who has significant history with Will. [Chicago Tribune]
An Abbotsford card shark has won the big prize in the latest World Series of Poker event in Las Vegas. [Canada.com]
A large contingent of Eastgate’s compatriots broke out into jubilant song in Danish when the young card shark laid down an ace of diamonds. [Sydney Morning Herald]