If the hypothetical thief already has physical possession of “your stuff” and is simply taking it away, I think it is problematic for the legitimate owner to introduce actual or threat of lethal force in order to regain immediate possession of “your stuff”. In most jurisdictions.
One would be justified in saying “Stop!” or physically attempting to restrain the thief or grab “your stuff” back. But that involves the gamble that you will be able to stay ahead (i.e. come out uninjured or at least alive) if the miscreant decides to initiate a force escalation race.
The same gamble (that you could lose the escalation race) will come up if the thief does not yet have “the stuff”, and one chooses to say “No!” or refuses to yield possession or access. Depending upon local rules of engagement, one might be justified a head start in escalation (i.e. display of weapon) if currently in possession of a residence or vehicle. Still the gamble.
Probably, it is best to decide for yourself — ahead of a given circumstance — whether it is more important to walk away, or to not yield. The bad guy probably did, and there’s no point yielding that advantage.