Missouri provided guidelines for the application of sales and use tax exemption certificates. Vendors accepting exemption certificates in good faith are not liable for sales tax on the exempt purchase. The certificate must be signed by the buyer and should list the claim for the exemption. Letter given by the Missouri Department of Revenue to the buyer exempting the purchase is acceptable as an alternative to a complete certificate. The vendor may entrust a buyer’s exemption certificate already on file for future purchases by that buyer until the certificate expires, is no longer standing in good faith, or is not applicable to the transaction in question. It is the seller’s duty to keep the certificate and update it every five years or until it expires to avoid being liable for sales tax. Accepting the certificate without good faith makes both the seller and buyer liable for sales and use tax. (12 CSR 10-107.100)