Hi dannyrichardwriter, i have some experience with a local site from my country. How well it sells depends of 2 things.
1-How awesome are your designs
2-How big is the market in the site.
One of my shirts sold well. It was on the top of their category for a while, but the market was small. the extra money was useful to buy some extra things i liked, like hearthstone packs, but not to make a living. (maybe with enough successfull shirts). Obviously it depends on the terms and conditions of the sites. In my case i chose how big was the commission, but the bigger i made it the more expensive they sell my shirt.
If you want to make merchandise of your comic, check with your audience how much they are interested, because people will not buy shirts of characters they don`t know. (unless they are really cute, because of a meme or "the che-gevara effect" is on.)
If you have the skills (judging by your profile photo i think you have them), go for it, but always read the terms and conditions.
Cheers