Try to find or buy an item with an enchant that will sell for a lot. Usually anything with Sneak, Banish, Archery, Pickpocket are worth a lot when you enchant them on junk. Then do the Azura's Shrine quest (and keep it) to farm bandits for grand souls to enchant that junk with. My first character made a pile from a lucky Banish weapon found in a chest. Just enchanted it on iron daggers with grand souls and sold them for usually the max gold a vendor had. Had 100K before I knew it.
I found Riften an nice place to fill the black star, first you have the ratway then the dungeon after the thief guild bar but still close to the enchant station, even more fun, during night you have common random event with thieves getting attacked by the guards, they are hostile so you can soul trap and kill them.
However the dark star is for your own or follower gears, not for sale, always nice to make utility items to, pickpocket, speech, carry more and other items are useful.