Why not "geen" instead of "niet een"?
Please see this thread for information on the usage of "geen" vs. "niet": https://www.duolingo.com/comment/3734833
I'm still a little bit confused. I read through the explanation in your link and it seems to support that the answer should be "Het is geen belangrijke markt". Any clarification?
Isn't it like in German? We can't say "nicht ein... ", we say "kein".
So in my opinion "niet een..." should be "geen".
But maybe I'm wrong... (I read the link but it supports my opinion.)