I’m not a very crafty person, but I know some people who sell things like that online. Ebay is a good place to start and I think Etsy has a lot of crafts. You could also start your own website and write some articles on your homepage that are SEO related and then have links to buy your products.
It depends on the craft, I think. Etsy is good, but they do take a percentage of your profits. If you can create your own website and manage transactions on your own, you might be able to bring in more income on your own. Any site that manages advertising and transactions for you (ebay, amazon, etc) are going to take a percentage of profits. Just depends on what your comfort zone is, really, and whether you’ll make enough of a profit off your work.
If you do graphic design for tee shirts and whatnot, there are tons and tons of sites that have regular contests for designs. Teefury is one. Threadless is another. I’m not sure what the going rate is for sites like Redbubble, but I’m sure there’s some percentage of profit for the artist.
I love to sew and have lots of experience and skills. But I cannot make enough money to make it worth my time. The only possible way would be for me to make high end products for which I could charge a lot.