You say this:

> I never saw this definition involving marketability anywhere, this is new for me. Perhaps it's really like you say, but that's not the definition I had

But then also this about your hypothetical about a “capitalist” system where “everyone owns capital” which makes it capitalist as opposed to socialist:

> You are free to manage it, sell it, buy it, as you wish.

You seem to explicitly recognize that marketability—the right to buy and sell—is an essential element of the kind of capital ownership that defines capitalism in the same post that you claim that it isn't.