I like the idea, think there are lots of places in town, where they would fit. If the style is a Tent or a Gypsy wagon don't matter that much.
They should work much like a Tent, with a secure box, that only can be accessed of your account. It should work like a community box.
Allow 2 vendors
Allow 3 mini add ons, like a forge, anvil and soulforge or maybe spin, loom and soulforge or maybe an oven and tables and bench outside to serve the food.
It should not be possible to make the wagon/tent private or close the door.
If you do not take it down, when the week are over, your items on vendors and in secure should go to the npc at bank, that also hold your stall items if you lose your stall and you have to go there and claim them. They will only stay there a week. Your gold on vendors should go to your bank.
The stall should be handled like the New Magincia stalls, just at this places, there are more tiles to the renter and they are placed near road, crossroads, near dungeons, at large empty places in town, at the edge of town. A few could be boats at large dogs like New Magincia. They should not be able to move.
All this could be a booster pack, so money to Broadswords.