In my limited experience with my own marketplace lot I made the tables become populated by NPC vendors. Unfortunately in my experience I was unable to use the tables myself ... I want to test it more though.
Seems like NPC vendors have their shifts, they leave after a while. I might make enough open space for my sim to bring their own table, though. Not tested it a lot yet.
Comments
Vendors tend to come and go. You can be a vendor if you find a free table.