Suppose we want to select a niche market in beauty products field (for example). which one is better to run a website? 1- whole beauty products? (for example a site about all beauty products. so in this case we can't cover all contens about all beauty products) 2- Or just one product and focus on it? (For example just lipsticks.)
If it is really unique product say for example magic cone one single product can work well otherwise you should go for few targeted beauty products range that best suitable for specific niche.
If you focus on only one product you'll save more time and you'll get more profit. Also you can offer related products in the back end. Just test!
I would go for smaller niche if I have enough content and resources. It is also easier to optimize smaller niche for Search Engines
Targeting individual products seems like the most profitable way to go about things. You could always end up creating a central website about general beauty products if the individual product websites turn out to be successful anyway.
Go small, the smaller the niche the better IMO. big niches get big fish. Realize how big a fish you are and go what you can get . 10% of a small market is better than 0.000001 of a huge one.
I'd say start small, conquer that niche, then grow it bigger. If it's a starting niche, then you can do well. I conquered a new niche about two years ago in about three months, held it for about 9 months then sold off the website. It works pretty well.
The smaller and more specific you can get your niche the better. For example, if you select one single product for beauty, such as face cream, you can create an entire site on that one subject, each page being a different article/howto on ways to apply face cream, when to apply face cream, etc. Don't make the mistake, like I have, of trying to be everything to everyone. Be specific, be creative, think outside the box. You could easily create a membership site based on one product, and promote other products on the backend (as someone else mentioned already).