My two cents in here on the original question.
So I know that these days potential forms can be transitive and take を because languages change over time - and in this case, maybe get influenced by English - but in my (old) textbooks it’s very clear that potential forms must take が, because they (originally) functioned differently than English potentials. To me their traditional use seems to mean something more like “to be —able”
Keeping that in mind, look at something like この本が読める. While a fine translation would be “I can read this book,” a somewhat more literal translation would be “This book is readable (for me, or whoever, everyone).”
Keeping THAT in mind, この本が売れる would mean that this book is sellable, which I think you can see how a natural extention of that could easily be that it “sells well,” especilly if you throw a よく in there.
Same with 知れる. “That is very knowable” = it is obvious.
That’s how it makes sense to me, anyway.