Now that's interesting timing.
I've just thrown some coin to a graphic artist to complete a design for me to do exactly that, well not exactly, but close enough. I'm going to release my design on clothing and in poster form and market myself through eBay, just using a local clothing printing company to do the work for me.
So I can't help you with a comparison between the competitors in that particular space, but I certainly see the potential market if you can really hit a specific market with your design and you market it as openly as possible.
I would never just leave it in a limited market like these companies offer, you do save cash upfront, but I don't think you give yourself the best chance for sales long term.
For my project, getting my design into a finished peice or artwork ready for printing is costing me 200 USD and it's quite an indepth peice of work. That's my only upfront cost. Then I have a short run poster printer who will run off the posters order by order, one at a time and ship customer direct within 48 hours and I have the exact same with my clothing printer as well.
So the trailing costs after the design all come out of the sale price at the time of the sale.
For my money, that's a better model to use and it gives you the freedom to maket where ever you like. The hard part, coming up with a design that hits a target market and spending a few dollars upfront. If you don't have a complicated design (like mine) then your upfront artwork can be crazy cheap as well.
Give me a shout out if you need a good artist to knock up a design for you, I'll pass on the details of the guys I am working with and they do some SERIOUSLY sharp work, so you have ZERO limitations to your end product in regards to design quality.
Just as a quick foot-note, I'm also really sus on the clothing quality you get with those companies. If you want long term success and repeat customers, clothing quality is a MAJOR concern. So another reason why I chose my own clothing/prining supplier. They may not be the cheapest, but you end up with a quality product.