What it is, how to get one, and why the seller is the one who has to produce it at audit.
A resale certificate is the document a US buyer gives a seller to buy goods for resale without paying sales tax. It is issued by the state, tied to a sales tax permit, and it is the reason a wholesale order can be untaxed at all. The part almost everyone gets backwards: the exemption belongs to the document, and at audit it is the seller who has to produce it.
It is a statement, on the state's form or one it accepts, that you are buying to resell and will collect tax when you do. It is not proof that you are a business, and being a business does not exempt you. A seller who takes "we are a retailer, trust us" instead of the certificate owes the tax themselves.
Register for a sales tax permit in the state where you sell. The permit number is what goes on the certificate. Most states let you register online in a day; a few take weeks. Then fill in the state's resale certificate form, or the multistate form where the state accepts it, and give it to each supplier once.
Keep it, check it is complete and current, and be able to produce it years later. States audit the seller, and an exemption with no certificate behind it becomes tax the seller pays out of margin for a sale to a buyer who is long gone. On the protocol a buyer holds the certificate on file once and every brand they buy from can see it is there.
The resale certificate is an American mechanism. Canada uses input tax credits, so the buyer pays GST and claims it back and there is no certificate. The EU zero-rates cross-border business sales under the reverse charge, against a validated VAT number. A US seller applying the resale logic abroad is the most common cross-border tax mistake in wholesale.