I'm asking this because I don't think I'll be able to keep up with the minimum sales requirement these guilds have (I play other games too), but of course I still need some kind of income to upgrade bag and bank space and such.
Pickpocket/steal stuff and sell to fences. If you want, be selective and go for green and above items (green sell for 100g and blues go for 250g). You can only sell a certain amount of stolen good a day, but there is a skill in the legerdemain tree that ups that number. A little time consuming....But it works.