I agree with Anton, it is very niche marketed business. I would like to add timing is equally critical, it is all about fashion and trends.
To vet T-shirt design and target market, sell your shirt at a big event, 3K plus, that attracts your target market. Interact, listen and observe. Profileyour buyers or reason for buying. Example, before conference you were targeting 13-17 year old kids. After conference, you realize, you do better with Male, 16-17, with families making over 50K a year. or Female that are into theater or Male that like body building. etc.
Once you vetted your target market fully, re-align site and marketing strategy. Just as a frame of reference, in the US, unless you selling 144 shirts of the one design within a sales cycle, you don't have profitable design. Good luck,