It is critical to buy from authorized dealers to ensure you receive authentic equipment with a valid warranty. CPAP Machines, Masks, & Supplies for Sleep Apnea | cpap.com
Device type (CPAP, APAP, or BiPAP) and specific pressure settings. Expiration date (though some are written for a "Lifetime").
You cannot legally buy a CPAP machine without a prescription from a licensed healthcare provider. Legitimate retailers will require you to upload, fax, or email a copy of your prescription before shipping your order.