It s not illegal as long as you don t claim to represent the brand that originally created the shirt. Just make it clear that you have altered the shirt and it was not manufactured that way.
Why would it be illegal? If you own the shirt, you can do whatever you want to it, and it s OK for you to sell your own property.You might run into copyright trouble if you try to do this with shirts that have copyrighted characters or logos on the front. Plain ones shouldn t be a problem.
Definitely more stupid than illegal. Far more stupid than illegal. That applys even if it was illegal.
No.. actually i ve no idea
