A resellers permit is something issued by the state that allows you to purchase products from suppliers without paying sales tax on goods the way a consumer would. Many suppliers also ask for a reseller permit to make sure you are a serious business owner/retailer.
In most cases, you will need a reseller's permit to do this business and buy from suppliers. If you are based outside the US, you might not need one.
Please visit the course section on licenses for more info on this.
Is it true that I need resellers permit in every state, as there are warehouses located in every state?