. . . then, in my experience at least, you have the choice of either taking an absurdly low cut of the sale price, or marking things up so much that nobody wants to buy them. It's fine if you want a couple of customized t-shirts or something (it was cheaper and more convenient than a short run of custom screen printing back when I used it), or are more interested in getting your image out than turning a profit, but it's awfully hard to make any real money with it.
That's just me being pessimistic, though, and my experiences were with a different company almost twenty years ago, so things may be different here and now.