To me "niche" sounds more like a term used to describe the market status. "Niche market" is one of the most popular buzz-words used in the MBA class. "Fist", however, is probably leaning towards the manufacturer.
"Marketable" could be a little redundant. The overwhelming majority of the off-the-shelf products are marketable. If I am about to say something good about a specific one, I would probably use "successful product", "phenominal product" and stuff like that.
Niche is a place in the market, it's true. But I think "niche product" means a product that can help secure a "niche" in the market. That's the logic behind "niche product" as something with competitive edge. Anyway your version may be accepted, too. There are always different ways of expressing similar ideas. Thanks for your response!