When someone rips off your design this quickly, and an original high res digital image of the design (color separated if it's a color design) is not readily available, do they just use a screen cap or buy one shirt and take the graphic off the shirt to make screens out of it???It depends on (1) how unique you are in your designs and (2) how your pricing stacks up. You also run the high risk of someone (particularly on eBay) finding and copying your designs that are successful and then undercutting you.
You've got to find a way to stand out / develop a niche for yourself to be successful on either one.