A seller's permit is essential to dropshipping from suppliers directly to your customers. Real suppliers will always require that you have a seller's permit in order for you to get a dealer account. You'll also need one to avoid getting charged sales tax on every order you place with your supplier. You will pass on the sales tax to your customer, which you will then remit to your state every year or quarter depending on your reporting requirements. This is different for every business.
To determine how to get your seller's permit, just type into Google "sellers permit [STATE]" and you should get the sellers permit for your state in the first few organic links.
You can also see the TaxJar.com blog for more useful resources.