How do I start a local market? (Fearless Blue)

How do I get my corps to trade among themselves?

You need to be member of Common Market. Either join one or set up one yourself. Once you become a member and use the Common Market interface it will become clear .. how to get your corps to trade among themselves .. as its kinda intuitive.

This said you can probably make more profit by selling your production on the World market and buying your supplies from there, rather than using Local market.

Ok, thanks. I'll try that.

Don't confuse the local market with the common market. You don't need to start a local market because it automatically exists when you start the game and consists of the trading you do with your state and national corps with either more of your own corps or your country and slaves. The common market is international.