Sourcing products is probably the most fundamental aspect of successfully competing online. Sure, key words, pictures, descriptions, etc... are important but if you can't compete on price then you have a big problem. You have two options in selling that include brand and price. Many times they go together and how you source these products will make a differences.
If you are a small business it can be extremely difficult to compete. Most things you find in China and the U.S. are priced low but unfortunately there are a lot of other people selling them. They are buying lots of them by the thousands thereby reducing the price to almost half of what you can get it for.
The other option is to source on discount. This means looking in stores and online for things selling under the market, buying them, and reselling them. Not easy as you are going to waste a lot of time finding products that are said to be on discount but are not cheap enough to pay for shipping or making a profit on price competitive Amazon.
You may also want to buy local stuff and resell it online. These are unique things that are not being sold anywhere else. Think of artisan crafts and productions that large producers can't copy. The type of person who buys this will be very different from someone who buys large production for cheap.
The other option is to seek high quality brands that you know will sell. For example, Camelbak is likely to sell more than a cheap off brand. People look for these brands and often type them in by name. The only problem being finding them.
So you have four major options that include bulk, closeout/discount, niche and brand. Each can work in their own way and all can help you earn significant money. That also means you will need to search high and low to find products that fulfill your strategy. Sometimes it is those places others haven't thought about that make the biggest difference.