Glad to have you with us Marie! :)
Generally the Sales Tax Id is required by wholesale suppliers based in USA, and this license basically tells the wholesaler supplier that you are a serious business registered to collect and remit sales tax from your buyers to your state. As Marc has mentioned this usually is not required by drop shippers. If you live outside the US you don't need a sales tax ID, but wholesalers may want to see some other proof that your business is legitimate. :)