Find a person who is an authority in that niche. For example: go to Amazon, type the niche (e.g.: dog training) and try to contact the person who wrote books about that niche and is considered an expert.
Otherwise you can visit: www.big-board.com (where you can find the largest message boards and forums on the web) type "dog training" and find who has the higher number of posts.
Try to approach these experts starting with posts on their forum and then you can send them direct message and even ask them an interview via web for example via Skype! If you use Skype together with http://www.pamela.biz/en you can even record the video conference and have other interesting tools. Don't be shy! Doors are made to be opened!
Imagine to interview 10 people who are authorities in that particular niche: BOOM! You'll soon have yourself credibility to sell a product in that niche!
If you don't know what to ask them because you're not an expert in that niche here a precious advice : go back to amazon, search the books about dog training or any niche you desire, click on the book and then on the "table of contents" where you will find the content of the book. Here you have a great resource to find the questions you may ask during the interview.
Try to chose 10 intelligent questions and they will be enough. Obviously when you write for example, you e-book, about that niche, you must put the name of the authorities you interviewed, after asking them their permission.
If you are gentle and brief they will appreciate you and you will have a great product which will give you credibility and will surely sell.
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