Corporate / Incorporation Lawyer
yes, as most professional sports team logos are trademarks of the team and this type of commercial use needs the authorization of the team, otherwise you could be infringing the team's trademark rights. Their also are some endorsement/fair trade concerns as you may be using the name to further the commercial sucess of selling the t-shirts
I agree with the first contributor. Please see the link, below, for information about what business people should know about licensing.
Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.
Yes, you need a license from the trademark owners to sell their products. In Los Angeles, you also need a business license to operate in the city, as well as a California board of equalization re-seller's license -- you may need similar things in your state.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.