I plan on doing this in the future, I did a lot of research on this, and interviewing people around me and other comic freaks.
I find people are reluctant to wear comic/cartoon designs on fashion-stuff, from what I've observed. No matter what comic it's from, those are just looked upon as childish, or just for "hardcore" fans.
mugs or stickers and buttons and such have a bit more liberty for people, though. I guess people are cool with smaller objects, but not something so conspicuous as clothes.
However, whatever it is that you're making, design is of greatest importance. Don't just slab characters or banners on white shirts and try to sell them. Design and style is important to people. (well, most people) I think the problem mentioned with shirts above has much to do with this. If it's got awesome design, it would be much more marketable. I've found in many comic-products the author just draws a character and puts it on a shirt. Not much else. Try to create a design that appeals to everyone, not just people who know your comic.
And don't sell just shirts. You said shirts cost like 20 dollars? (Yeah I did some research on this as well, and that's about the cheapest price I could find) You'll find that having variety of products and designs will get you more buyers. Zazzle and Coffeepress and most other companies print on demand, nyways, so you dont even have to worry about having too many products and designs. People like having choices, whether they'll take advantage of all of them or not.
AND if you're gonna use zazzle and such, also make non-comic related products, too. They won't go much promotional for your comic, but it is definitely getting you traffic into the "shop." They can look around and if they find something else they like, they can buy it.
edit: I recommend you starting this after you're sure you've got a good fan base. Or else it's just high hopes and frustration. Among people who read the comic, there are very few who actually buy these things...
another edit: I just found puzzles on zazzle. I love this. I love puzzles and I love comic. This is so much love.
And yes, this actually got me another thought. Products that people can "do" something with, rather than posters and buttons that just sit there, sound more fun. Just my opinion. :)
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