Karma predates Buddhism by a long shot. The Buddha was born into a culture that already took karma to be settled fact.
While you can construct an elaborate system of beliefs about how you think karma works, at its core karma is a very basic general premise that most humans intuitively accept: intentional actions have moral repercussions.
This doesn’t require a great leap of credulity. If I knowingly take an action that harms others in order to serve my personal interests or out of hatred, then on some level (maybe we could call it the moral sphere) things have taken a turn for the worse. It’s why most of us understand that waterboarding isn’t a good idea, for instance. That’s karma.
The perpetrator of the action, even though they may have seemed to profit from the action in the short term, is now a morally compromised creature, more psychologically isolated from others, and that’s a recipe for suffering. On top of that psychological retribution, the perpetrator also has to live in a world made worse by their own action and they can’t be entirely immune from those bad effects.
When you extrapolate from this basic understanding of karma into a belief system that purports to plot out how all of this works, then you’re on largely speculative ground.