There are two types of online t-shirt shops. The specialized and the "all you can eat" and mixed.. so three kinds I guess. That's clearly simplified but basically true, a smart way? is to build your own site so you can switch between them if things change. An example of this is The Depresident Shop which currently is on Redbubble's system.
Yuuuuge site with a lot of stuff. Many of the best designers have left b/c of poor earnings potential.