A sales tax exemption certificate is needed in order to make tax-free purchases of items and services that are taxable. This includes most tangible personal property and some services. A purchaser must give the seller the properly completed certificate within 90 days of the time the sale is made, but preferably at the time of the sale.
You may use an exemption certificate if, as a purchaser:

you intend to resell the property or service;
you intend to use the property or service for a purpose that is exempt from sales tax; or
you make purchases as an agent or employee of tax-exempt nonprofit organizations or government entities

As a purchaser, you must use the correct exemption certificate, and complete it properly before giving it to the seller. The exemption certificate must include all the following:

the date it was prepared;
the purchaser’s name and address;
the seller’s name and address;
the identification number on the purchaser’s Certificate of Authority (some exemption certificates don’t require this, or may require another identifying number; see the certificate’s instructions);
the purchaser’s signature, or an authorized representative’s signature; and
any other information required by that particular certificate.
If you willfully or knowingly issue a false or fraudulent exemption certificate, you may be subject to penalties, fines, or a jail sentence.