To answer your question: high schoolers and college students in the sciences or engineering. Or generally anyone geeky enough to buy this tshirt. That's why I went to HN first. It didn't explode like I fantasised it to, though.
I didn't buy a bunch of tshirts hoping to sell all of them. I'm selling the tshirts on Teespring. Teespring handles everything for me. They design, print and ship the tshirts. Only if I get enough pre-orders though. I just get the money at the end of the campaign. :P
I can definitely see this working on the street. In retrospect, I'd buy one on the street if it was presented to me. But I don't have the time to do this. I'm just a lazy college student hoping to make some easy money.