Is vistaprint and zazzle really necessary?

Another forum refered me to vistaprint, and I came across zazzle on accident I think. Is this really a good way to market my art? I am doing a lot as it is, but managing and keeping track of an online shop seems like another burden. Although, if it is really a good marketing tool, I'll be willing to do it. The big issue is the fact that I have to spend money for each design being put on something like a T-shirt. I just thought I would run it by a couple of freelance designers before I get into all that. Anybody have experience with these sites?