Hi, I believe if Tee's belong to a brand, you'd need their permission to further follow up and trade with.

However if private label then again you need do your research and find out if you should narrow down your search to choose a sub category or micro niche.

You can use Jaaxy to drill down keywords, look for searches (avg.) above 30 and competition (qsr. ) lower than 100, the lower the qsr the better.

https://my.jaaxy.com/search/keywords

For your choice of keywords, always do an incognito search on Google before settling on a keyword, to see who appears in the top 3 positions.

Further info and resources that may help you out include:

A niche is something you are passionate about and would write 100 articles or so about it, could be a hobby, an interest perhaps. To put it in perspective, would you be writing about it in five years time.

Via Kyle:
A niche can honestly be anything, what are you passionate about? What are you interested in?
When deciding on a niche and trying to narrow down to a more specific niche, I recommend you ask yourself the following question:
I want to help people that ______________.
Fill in the blank and you are going to be much closer to a specific audience. After you do this and if you want to run any further ideas by me, let me know and I will give you a hand. :) A Niche is A Group of People Looking for Stuff If you are New and Not quite yet figured (niche infographic within) Choose a Business You Love, Not What You Know. Succeeding With Golf Balls, Scaling With Shoes! How to Find Your Niche With 5 Google Research Tools Kindly let us know how you get on.

Not really sure, but I would not do it, just too much work and the profit margin are low.

Design work, they like, they do not like it, you will be dealing with too many idiots before you have a decision with your T-Shirt.

I would think twice.

Joseph