Pretty much what Ennoch said. Selling people (just quicksell unless you think the slave is worth a lot at auction) will fund your investments into other stuff, including a workshop you can put to use cranking out rifles.
They sell for more than the market at the trader in Hamah Bay, so make a ton of them and go sell 'em there every once in a while.
Donations to Aesir, Catholics and Muslims will also provide additional passive income. Only issue is Aesir require you being Aesir to donate, but they take slaves, Catholics take food (look into Zapin Fields) and Muslims take merchandise, once you have quite a few scavengers working.
Also, don't forget to hire Armand to be your accountant once you're making enough from investments.