Unfortunately, many have no idea of the value of their used machines, which is why you'll see a crazy range of prices - you can often see people asking more for their used, non-data capable machine than you can buy a like new machine from Supplier 2. They only know the original, pre-insurance cost was a couple thousand or more, so they figure it must certainly be worth $600 or more. Many, if not most, of the Craigslist ads for cpap machines are asking crazy high prices.
But not all. In the past I've purchased a lightly used S9 autoset machine with warranty from Supplier 2 and been very happy with it - they even replaced the humidifier about 6 months later when it went wonky, no questions asked. Excellent vendor.
More recently I also recently jumped on a lightly used S9 autoset on Craigslist for $140, AND the seller threw in several almost new masks of the exact type I prefer, several brand new unopened replacement pillows, and several new, unopened hoses. I would have had to pay more than $140 even at online discount vendors just for the mask supplies.
The key was I knew *exactly* what this machine was worth, how to check the usage hours, and I immediately jumped on the ad when I saw it. Great deals like that are rare, and when they pop up you need to already be informed enough to recognize and quickly move on the deal before someone else does. The great deals go fast.
So deals are definitely out there if you know exactly what you're looking for, set up alerts so you know about new ads that might meet your criteria quickly, can be patient in waiting for the deals, and act quickly as soon as a good deal appears.