Don't quote me on this because I'm not the all powerful goddess of rule knowing, but there's a guy who I bought a couple of Touhou T-shirts from over the internet who sells them at touhou and doujin conventions and he says it's fine. If you're going to sell them, however, get an original artist to do the design for you, you don't want to be selling t-shirts with other people's art on them when it hasn't been authorised by said artist.