I know there are already a ton of t-shirt places online and room is sparse, but I've designed a couple of my own shirts and friends and have had a bit of a local demand for them.
I can get into details/examples later, but for now I'd rather leave it at that.
The main issue I'm facing is how to get these printed. The ones I've done so far were just for fun and I used iron-on transfers which, needless to say, do not last or look the best. While I have no issue using them for myself or giving a shirt away to a friend... I couldn't sell that without feeling guilty or putting up a bit warning lol.
I know of a local guy that does screen printing with no minimum orders or setup fees, but he charges $2 per color per side, and the average amount of colors could vary from a few to several making it not worth it.
I figured the next option is full-color t-shirt printing... but I have no idea who is affordable and nice quality. Many of the online sites I've seen produce horribly colored/quality shirts, so I can't say I've ever heard a good review for one.
The main thing is there can't be a minimum fee or setup fee as these shirts will be personalized and I therefore have no ability to recoup the setup costs in bulk sales of a unit.