At or in the market both mean the same thing. However I haven't heard on the market used in that way before. Both the stalls can be described and at or in the market. On the market would normally be used to describe the market as availability of items rather than a physical place. Ie the house is well priced within the market. It was on the market for 400,000 dollars for example.

- They have a fish stall on the market. - "On the market" means the fish stall is for sale. - He has a flower stall in the market. - "In the market" means there is a market and the flower stall is found inside. This might be a large grocery store. - They have a flower stall at the market. - "At the market" means there is a flower stall that can be found in the same location as the market. This market might be open and a collection of sellers that are outside of a main building.

In these examples they mean the same thing. 'On the market' is least common and doesn't work for all contexts. You can say, 'I'm at the market' or 'I'm in the market' but you cannot say 'I'm on the market'.