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...
Actually, I'm pretty sure you have it the wrong way around. Supply comes from the fact that there is value, with value in in itself being derived from the fact that there is demand. Supply does not dictate demand, instead suppliers will move to fit the demands of the consumers. If no one in the world demands your signed t-shirt then it has no value and then no one will bother to supply it. If there is demand, someone will eventually find a way to supply such a demand (provided it is viable).