indentityburn, you are correct. But as the OP stated, MN has no sales tax on clothing. So even if someone in their state bought shirts, they would not have to collect the sales tax.
But, amk2u, I did think of another issue for you. You don't have to collect sales tax on the sale of your shirts. But what about you paying taxes on having your shirts screen printed in MN? If you have a state resale number, you wouldn't have to pay taxes on the services provided to you that are for your business. This could save you some money, especially since you can't charge your customers sales tax.
Let's say you get some blanks wholesale from somewhere, and you don't have to pay sales tax. Then you want to get your shirts printed in MN. If you don't have a resale number, that screen printer is going to charge you sales tax on the services that they provided for you. That increases your costs. Which you would then have to eat, or pass along to your customers. But if you have a resale number. I'm pretty sure you wouldn't get charged the sales tax because it is for your business. I know here in TN, I don't get charged sales tax on goods or services because I have a resale number.
Just to help you out (and anybody else that's interested) here's a link to a site that has links to all of the different states' dept. of revenue websites. Just click on the flag of your state and it will take you there. Most of the sites are very thorough, and should answer most of the questions you might have.
http://www.nass.org/sos/sosflags.html