Maintenance for the week of March 16:
• PC/Mac: No maintenance – March 16
• ESO Store and Account System for maintenance – March 18, 6:00AM EDT (10:00 UTC) - 4:00PM EDT (20:00 UTC)
You can only sell stuff through guilds that you are part of. You can go to any banker and talk to them. The option to see the guild store is listed. Then you pick the guild you want to list stuff in.
You can also sell by talking to your hired trader directly. It brings up basically the same interface you see in the bank. This means you have to port to them in the world, though.