Greetings! I started out on Earthen Ring as well. I have hosted taverns in Boralus and around Kul Tiran most Fridays for the past two years, catering to the Alliance. ★ {A} Jac's Bartending
There is Tautstring’s Kul Tiran/Tradewinds Markets on Sundays for Alliance in Boralus, which were on a long hiatus but will return on the 18th. There vendors can set up, much as you say. There used to be Stormwind black markets at the harbor put on by The Coldwall Coalition, I do not know if those still occur. I believe there are various cross-faction and Horde events like this as well. These Tumblrs regularly advertise events which may point you in the right direction: