But the Danish sentence contains the word "torvet", and in English this would be "the square". A market - in England and in Denmark - can be held in a field or on the town square. Apart from that, a solitary fishmonger could acquire permission to sell fish from their van on the town square, and that wouldn't constitute a market.
As with many danish words there are many meanings, and so is the case with "torv"
- større, åben plads i en by, typisk belagt med fliser eller brosten og udsmykket med fx et springvand eller en skulptur undertiden centrum for gadehandel (<- town square)
2 åben (overdækket) plads omkranset af butikker, fx i et butikscenter (<- strip mall?)
- sted med handel, ofte fra boder (<- market) Synonym marked
So yes, technically you are right, the fishmonger could be standing alone in the middle of the town square trying to sell fish out of a basket by his own lonesome. However the sentence said "They sell fish" so there would at least have to be more than one person trying to peddle fish. Perhaps he brought his wife? :-)
My point remains that using "market" as a translation for "torv" should be accepted here.
Looking at a more practical real-life example of the use of the word "torv"; one of the largest venues for buying and selling fresh produce in Denmark is Grønttorvet (http://www.groenttorvet.dk/). On their English section of the web site they refer to themselves as "Copenhagen Wholesale Market".
Yes, it's all true what you mention above, but I maintain that "torvet" is not synonymous with "market". Grønttorvet in Copenhagen is not a torv as such, but is a permanent market with the name "Grønttorvet". Whenever a person in Denmark would mention "torv" to me, I would never assume that they meant "marked". As for the plural "they sell fish", well a fishmonger need not neccessarily be just one employee, but two or more in the same van or shop. This still wouldn't make it a market. One more example in Denmark is "Torvet i Silkeborg". During the week it is just a town square, but every saturday they hold a market there which they call "torvemarked" but never "torvet" refering to the market. https://da.wikipedia.org/wiki/Torv
I spent a lot of vacations in Lemvig, Denmark. The area named "Torvet" is not at all a square but the place all around the church where the market is held once a week. I really think, from that stand point, as well as from the other comments I could read, that "market" should be accepted as well. I was extremely surprised to fail on that one !