Wow how could Marx have gotten it so wrong? Capital can be used to purchase goods consumed by the self (as is the case with almost everybody). Or to collect as collateral against risk. Or to invest. Or, really, all the real reasons we use it.
You seem to be confusing capital (the means of production) with money. Money can be used to buy capital (in a capitalist economy), and due to that is frequently used as a measure of the quantity of capital in such an economy, but is not the same thing as (or even a subset of) capital.