In modern society, farming sea life is a pretty common activity. Oysters, pearls, seaweed and all manners of sea based resources are farmed by people on land or in a boat and rarely divers.
The main issue isn't access to these underwater resources, its that sea water will naturally corrode and destroy and equipment used to harvest, store and process the materials when applied at scale.
Modern farming means modern machinery, and since sea water corrodes metal 10x faster than it sitting in the air, your Merfolk are going to have an insane amount of upkeep to constantly replace the equipment they need to compete with modern farming. Not to mention the rest of the manufacturing process.
So your Merfolk might be able to maintain a nice and profitable boutique Sea-Silk operation, Or they could be a Cog in the machine that is the global consumer market that will harvest millions of pen shells in one country. Ship them across the world to be processed in another. And ship them back across the world to be sold.