Open Discussion: How do you determine high selling items within a niche?

Hi guys. I thought it might be interesting to share methods on determining high converting products within a niche. Some niches have thousands of products and tons of brands, and it makes sense to concentrate on the highest converting of these. Otherwise, you could spend tons of time and money on ad spending, SEO, marketing, etc... on products that just aren't in demand.

I'll try and keep this broad, but feel free to share more personalized research that you do. This isn't about niche selection - it's about selecting profitable and high-converting products in a large niche once you've selected a niche.

Here are my primary methods:

1) Amazon.com - I sort by "New and Popular" and "Avg Customer Review" in a specific category. I usually take into account the brands that appear and the number of reviews and the average rating. I use the number of reviews submitted as an indication of how much the product sells.

I assume, though not very scientifically, that a certain percentage of people review a product, and that this percentage is relatively consistent across the niche. Products with higher reviews submitted would thus have more sales. Seems logical, but I'd be interested to hear others take on this assumption, and if there is a more accurate way to determine it.

2) Google Shopping - With a list of manufacturers on hand, I type their names in one at a time and see which of their products appear up top, and which ones have the most reviews (and the highest rating).

3) Competitor Websites - I visit the big dogs in my niche and scope their site to see if they have a "Popular" or "Top Selling" category.

4) Keyword Research - I usually use keyword research as a supplemental tool, to confirm that the products I've found are sought after.

I've wasted tons of time and money on uploading, marketing, and advertising products that never sold, simply because the market didn't like them. The take home lesson for me was to follow the ol' 80/20 rule when it comes to your product selection within a niche. Upload the 20% of products that are going to bring you 80% of your revenue. Then worry about the other 80%.

I'd love to hear your take on my approach, or how you approach this research yourself.