Value doesn't come from demand, quite the opposite. Some value should necessarily exist before to provide. And the intrinsic value you refer to here is called utility, i.e. usefulness for achieving individual's ends. How could value be intrinsic if it changes with people's preference?
Surely value comes from demand? If I own a signed t-shirt, and 1000 people want it because the person signing it is famous, its going to be worth far more i.e. more valuable than if a nobody signed it and only they wanted it
I don't know whom you replied actually, but no, demand comes from value, not the other way round. And your case just confirms it. People would value signed t-shirts, and thereby arises demand for them...