Retailers who purchase products for resale without paying sales tax must provide a resale certificate (reseller’s license) to the wholesaler.
This document proves that the retailer is registered with the state’s taxing authority to collect sales tax.
If a retailer fails to provide a resale certificate, a wholesaler can (and should) refuse to sell to them without also charging sales tax.
A resale certificate is often the very same document as a sales tax license.
Just like with everything sales tax related, resale certificates and the laws surrounding them vary by US state.