Reputable sellers must ask for a prescription. If they don't, they are likely operating illegally and bypassing safety standards.
These authorized online retailers specialize in certified pre-owned and refurbished units that have been cleaned and inspected: Is It Safe to Buy a Used CPAP Machine? - Sleep Foundation buy used cpap
Never use a second-hand mask, tubing, or water chamber. Always buy these brand new to ensure sterility. Safe Sources for Refurbished CPAP Machines Reputable sellers must ask for a prescription
Only purchase from authorized medical retailers who professionally sanitize, test, and repair devices. and repair devices.