Established markets are markets where products exist and where you intend to compete by offering a product that serves a niche within the existing market or is a complement to an existing product.
Consider attacking the low end of the market first. Customers at the top of the market aren't willing to risk their business on an unproven startup. In contrast, customers who are fighting the big players in the industry are constantly looking for an edge and are more willing to work with a startup that could give them an advantage.
I’ll sleep when I am dead.