He connects sellers and buyers, just like a grocery store does. He gives business to t-shirts producers who wouldn't necessarily be able to chose what t-shirts

He connects sellers and buyers, just like a grocery store does. He gives business to t-shirts producers who wouldn't necessarily be able to chose what t-shirts to make, to market them, charge for them, etc. And he helps buyers by giving them a convenient way to order and pay for t-shirts.