If I capture Elon’s roadster in space, would it belong to me?

Interesting question. If you can pick up trash, are all the "defunct" satellites also fair game? How about the operational ones?

Wouldn't that be a hell of a prospectus for a satellite communications company: "We're going to eliminate our competition and control the market through physical dominance." I can hear the Venture money getting erect already.

yes. Hypothetically anyway, because who is going to spend a billion dollars to come after you.