The singles are for consumption and turn in points. A set of the doubles is easy enough to collect if you put some time and effort in and they are great deco. A set of the triples is an accomplishment and can take quite some time to gather. Oddly the doubles and triples have ZERO turn in value and thus are good for nuttin' but deco. I have never seen a set of triples for sale, doesn't mean it hasn't happened obviously. If I ever complete my second set of triples I'm going to throw a fulls set including singles, doubles, and triples in an auction just out of curiosity.
To my knowledge the only place to get them is as loot from merchant's and orc pirates. So anyone who doesn't bother with ships and cannons can only get them by buying. No idea how big of a market that would be. How many folks won't/don't bother with high seas stuff and what % of them have a desire for a collection like that? *shrugs*