For a pirate game, one thing you wanna curb is players making all their money just selling captured ships, since it can lead to ridiculous amounts of money coming into their possession very quickly.
Luckily, you can solve this by taking some lessons from reality. By maritime law, all ships must be registered in and fly the flag of a country. And most countries aren't going to be willing to register some ship that has neither proof of legitimacy from the manufacturer or transfer of ownership from the previous owner, so, costly as they are, merchants wouldn't be interested in a captured or stolen ship, as they wouldn't be welcome in most ports (if you happened to come across an abandoned ship or captured a pirate ship, port authorities might be grateful for you surrendering it under their custody, and might be willing to sell it to you, maybe even at a discount).
Meanwhile, pirates present a much smaller market than merchants, as there are less of them, and they will usually already have the ship(s) they need or capture or steal them themselves. If you have a particularly nice ship, someone might be willing to buy it, or they might try to obtain it by other means (though the Pirate's Code will probably stop them from outright stealing from other pirates in most cases). Otherwise someone might be willing to buy an unregistered/untransferred ship for maybe or maybe not legitimate purposes at a significant discount.
If players really want to make money capturing prize ships, they'll need to get some sort of letters of marque.