You can define capital as "stuff" if you want, but that's not useful to any serious discussion or analysis of the actual existing system of capitalism (but I must say that Capital itself is well served by people who essentially see everything around them, and even inside them, as potential capital).
But it's ok, we can go with your definition, I would still say capital is an idea that we need to be rid of entirely. The concept of capital enslaves us.
#capitalism is not voluntary at all. It required #Enclosure and #colonialism for it to be born and it continues those violent processes to maintain itself.
Wealth does not come from stuff, it comes from the fences around stuff that we specifically need to survive. Capitalism leaves us no option other than market society. "The goods you are expected to buy are made out of resources you have forcibly been excluded from using yourself" ("The Prehistory of Private Property" by Karl Widerquist and Grant S. McCall). It is an arrangement of coercion, not voluntary choice.
Wealth is a measure of social power over others.
Here is some perspective on market society: