What's important is that you are buying from a legitimate supplier of these designer clothing brands. You don't necessarily need a license to sell. But do try and secure proper documentations from your supplier, such as Certificates of Authenticity as back-up in case you may need to prove that the items are indeed authentic as claimed. You may need this for lets say custom issues or other instances where you may be required to prove that you are not selling knock-offs.
Hope that helps. Cheers :)