With all taxable sales, sellers are required to collect sales tax, an exemption certificate, or a resale certificate.
With the nature of how drop shipping functions, it’s almost guaranteed one party is functioning as a reseller. For instance, a retailer who purchases goods to be delivered to the customer is the reseller. They’ve already collected the tax on the purchase price, and would not owe additional tax to the supplier.
However, the retailer must provide the supplier with a valid resale certificate in lieu of tax. If a supplier doesn’t have proof of the exemption, they’ll be out of sales tax compliance.
The same holds true for any qualified purchases by tax-exempt groups. A valid certificate is required to verify the purchaser’s status, and maintained in case there’s an audit.