If you do themed drawings for boat shows, home shows and other events it might be a good thing to consider. The portable canopy and inexpensive Bungie cords with inexpensive tarps makes a nice shelter for sales. See how others do it at a show.
Discussion
There are a lot of illustration markets in London, but a stall at one of them can be costly and you need to invest a lot upfront to create prints, zines or whatever it is you’d like to sell, and invest in a stall set up to grab attention too. I just want to sell a bunch of sketches I have so an online sale seems to me would be better for that.