According to the pros at Amazon, there are some common factors in t-shirts that sell well:
Avoid simple white t-shirts.
My gut was wrong on this one. I thought fer sure that simpler white t's would sell better than dark t's, so the first round of shirts I designed were mainly white. When I thought about why I believed lighter shirts would be more desirable, I realized it was because I personally prefer white t-shirts. I feel like they are lighter not as stuffy for the humid Minnesota summers. This is an example of an egocentric marketing thinking error - thinking what I like is what everyone likes!
https://www.amazon.com/dp/B072KJ3QR5
To get the most sales, Amazon recommends the following:
Dark shirts with strongly contrasting colors in the design.
Designs that are clearly visible as thumbnails to online shoppers. To make the designs easily visible, stick to simple, larger designs. The more parts there are, the smaller the detail and the more difficult to see.
Include a brand in the design! If you are pubbing a brand, weave it into the design, or simply design the shirt to carry the brand alone. If the people who know the brand have some inside jokes or unique lingo, try some shirts with those phrases.
Create a character to go with the brand and include that on some shirts.
Best of both worlds - build a design that subtly communicates the brand so insiders will recognize it, and also appeals to those not privy to the brand...sneaky!