First of all, the most important thing to remember is that when you need a *Real* legal opinion, to go and get some real world legal advice

Ie: If you were thinking of one of your own potential print designs and wondered if it breached copyright, that's the time to go see a lawyer
In the interim, there are a few different ways in which one can sell shirts with someone else's property on them. The first, and most prominent with the Nintendo themed shirts, is licenses. You see a lot of them these days, Super Mario Kart, Mario World, etc etc - those are licensed for use. Theres 80stees.com that sells quite a lot of these, and other such sites. There are huge companies that go and acquire the licenses for 100's of different images and then subcontract them out to printers, or sell them wholesale themselves. That's the reason you've always seen Batman/Superman shirts around.
Another way is to border on copyright but fly under the radar. Ie: Printing your own graphic accompanying copyrighted lyrics, band name, etc. and selling them to your friends, at markets, etc. I've done this (tee-hee!) on the low-scale of 20-30 shirts, selling them to friends all attending the same festival and wanting matching shirts.
Then you can go all out and breach copyright and stop only when you get a cease and desist letter. That guy that sold all those Dumbledore is Gay shirts a while back, that was pretty much his approach.