I did just a bit over $1 million in sales over two years of my Amazon business and it was 99% existing major brand lines. Some I bought direct from manufacturer, some from other suppliers.
For me, it wasn't so much identifying top selling products, but rather ones that I could compete on price. I did use a good many tools for all aspects of the business, but couldn't really share them now as I've basically shut all that out of memory.
The night of that worthless settlement I started looking for a job back in trucking, there was a job fair for a local company the next morning, and I ended up there for 1.5yrs.