If you've considered a reselling business as a side hustle, or are just curious about the concept, this one's for you. This article is going to be focused on sourcing, that is finding stuff out in the wild to sell, specifically at Estate Sales. My own chosen niche is books, but much of the following article could be adapted and applied towards other items or niches.
Here's the thing- Becoming a polymath is all about balance and relativity. Figuring out the exact best path for yourself is a journey that only you yourself can undertake. Pursue too many things at one time and you'll spread yourself too thin, only developing a cursory knowledge of subjects. Focus too narrowly and you'll lack the breadth of knowledge that is so characteristic of the renaissance person.
David Epstein's book is solidly grounded in the non-fantastical real world. It makes the case that people who have experience in a number of different areas (generalists) have some advantage(s) over those with deep expertise in a single field (specialists). This is the case even as many career fields insist that their professionals get ever more and more hyper-specialized. This can cause those specialists to miss new ideas or connections to other fields which would have been beneficial.
What I like about the advice given in Essentialism is that it isn't some trite quick fix. Instead, it seeks to give the reader a foundation for determining what is actually worth devoting time and energy toward. Spoiler- not most things. If you've been into the self-improvement niche for a while, you've probably heard the phrase, "If you don't decide what to do with your life, someone else will" or some variation of this. But how many of us continue to say yes to our boss, our friends, etc when offered yet another project, another "opportunity", another activity?