What about using one of these sites like Zazzle where you upload designs and then can earn a commission on every item sold through the site for your design?
I often go looking on them for geeky kids' t-shirt designs and they seem to be quite easy to use, I even know a graphic artist who has made a few quid uploading designs. The bonus is they will print to a multitude of items/sizes/colours - you can have mugs and t-shirts, you can even have baby bibs! There might be a market for those with this being a fertile website
There are other sites that do this of course, and just a suggestion - struck me as potentially useful as then nobody needs to store 200 t-shirts in the garage and post them out. It seems to be minimal effort - designs are uploaded, purchased and the BW sits back and checks the bank account?
EDIT
Cafepress.co.uk - that's the British one, the design tool is fun to use.