Product research is a key component for businesses to decide what products to sell. There are many resources you can use to find popular product niches. Pintrest is a great website to start to get ideas of products people are into.
Ebay has a free research tool called Terapeak. This provides valuable data on your competitors. You can search for top selling products and gain an understaning to how well they sell.
Google Trends is a free tool provided by google giving you insight into search data. Take a look at what people in a specific country (or worldwide) are searching for on Google. This will give an idea into what products may complement those searches
Amazon provides a Best Seller Rank for each of the products listed. Again, this can give you a good idea of how popular that product is.