When you look at used cpap make sure to ask about the blower hours not just the patient hours this is especially true when looking at respironics machines .For Examples of the machines and Auction Prices 

CPAP - Auctions.com  . Make sure you purchase Clean Filters and a NEW hose and Mask . If you are aware of what you want for a pressure be sure to pick up a clinicians manuel for your machine in order to program it to your Doctors prescribes pressure. If you are really in need of cpap and can not afford to go to a sleep study your best option is to get the doctor to prescribe an Auto CPAP . This machine has a computer in it that will adjust to the pressure that your airway needs to keep it open through out the night . It is the next best thing you can do beside  going to a sleep lab and letting them figure out what pressure you need to cure your sleep apnea. If you use a cpap with to low of pressure it may not open your airway and make it even harder to breath when you have an obstructive apnea . To high of a pressure and you may not be comfortable and have a large amount of mask leak . You want your pressure  to be just right .  If your pressure that you need is higher then 20 cmH2o then you need to get a BIPAP . A BIPAP can be set to do CPAP at pressures above 20 Cm H2o. Pressure above 20 CM H2o are for extreme cases of Sleep Apnea but it does happen.