Comment on
What are the First Principles of investing/personal finance?
Buy low, sell high is applicable to trading securities which is only a minor part overall of personal finance.
Personal finance in its whole is a system for managing the flow of one’s money in an optimized way which both minimizes losses to taxes and interest while maximizing ROI.
A great starting resource is the Bogleheads wiki: https://www.bogleheads.org/wiki/Getting_started
They provide general flows to managing personal finances.