When I built my first company, I targeted a niche and was able to corner it. Is attribute the success of the company to a few key factors: helpfulness, consistently excellent service, and high relevance.
The best way to get the support of a niche is to be truly useful to that niche.
Where do they hang out online? Be there, and help everyone you can with no expectations. When it makes sense, link back to relevant resources to give them the option to research more deeply.
Keep a blog full of helpful information — this helps the resources you link to when helping more useful to you, because they're yours.
And — most importantly — make damn sure the product or service is filling a gap in the niche. All your efforts are for nothing if what your selling isn't wanted or needed.
Then you simply have to make sure you're treating every customer like a celebrity endorser. Their word will make or break you, so make sure every client is your most important client and that they finish the project ready to tell everyone how great you are. "Pretty good" won't corner a niche; "fucking awesome" will.
If you'd like to talk strategy and/or marketing, hit me up. I'd really enjoy working with you on a project like this. :)
It depends on how you plan to sell the items. If you have a eCommerce store you are looking to sell on then it is all about content. Organic SEO through valuable content - text and video is Key.
Selling the items on Amazon and Ebay are the easiest ways. You just have the list the product with appropriate key words. You could then expand from there to an webstore after you have proof of concept and some revenue coming in.
For new niche items Amazon is the best option. For used or collectable items Ebay is the better choice.
Create a marketing strategy that provides a strong ability to retain clientele loving your mindset. Social media, e-mail newsletters, etc. are a wise move. Don't seek out instant success, it will be a slow drip.