Robbing other players is easy and highly profitable, especially if you're demon, but, I advise everybody to invest some land in thieves dens. You can steal Gold (obviously), Lumber, Iron, Adamite, and Food. Stealing is very easy, simply go to the coven listing, find an appropriate target, click their name, then click where it says thieve (it'll be at the bottom on the right), select what you want to steal, chose to send a hero/prince/dark knight, and click thieve.

Robbing more than 10x a day or 10x a day every day is bad news and can garner sanction.