The issues on the North American and European PC/Mac megaservers have been resolved at this time. If you continue to experience difficulties at login, please restart your client. Thank you for your patience!
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.