After you have successfully seized a few ships’ cargoes and raided a few villages, your ship’s hold will likely be filled with plunder (typically, 1 point of plunder takes up 10 tons of a ship’s cargo capacity). Until you hide this lucre somewhere or take it to port to sell, you will no longer be able to take on plunder in any significant quantities.
The best options for undertaking such a task are the neutral ports of Bloodcove and Senghor on the Mwangi Coast, or farther south, the Sargavan port of Crown’s End. Bloodcove and Crown’s End regularly cater to pirates, slavers, and smugglers, while Senghor at least turns a blind eye to pirates, so long as no piracy takes place in their waters.
Senghor does not tolerate slavers, so if you have slaves to sell, you would be well advised to seek out a different market.
All of these ports are accustomed to servicing pirate ships and can provide all the services for repairs, supplies, and recreation that you and your crew might need. Bloodcove is a small city, Senghor is a metropolis, while Crown’s End is a small town. You can opt to sell prisoners as slaves or have them held for ransom (the latter managed by ransom brokers available within the cities themselves).
Unless considered as plunder, individual captives can be sold as slaves for 75 gp each if uninjured (half that if below half hit points). Officers and other important NPCs can be ransomed for 50 gp per class level, after the brokers collect their fees.
But if you show up in Bloodcove, a notorious Aspis Consortium stronghold, with Aspis Consortium prisoners or an Aspis Consortium ship for sale, there will be trouble.