
If you buy parts or materials to mark up and resell to customers — copper pipe, light fixtures, hair products — a resale certificate prevents double taxation. Without one, you pay tax to your supplier AND collect tax from the customer.
No. A tax ID (EIN) is for income tax and payroll. A resale certificate is specifically for sales tax exemption on inventory.
Generally yes, if you have nexus and buy inventory in multiple states. Some states accept the Multistate Tax Commission Uniform Certificate.