I recently bought a PRS p550 off Craig's list and I was lucky. There were 4-6 listed. I responded to each and asked specifically what model since most ads were vague; and how many machine hours not therapy, not blower hours but machine hours. Only 2 replied so I assume that their words about low usage were not accurate. The ones that did not reply are still advertised so they had not been sold.
I would not hesitate to switch back to a fixed pressure machine in an emergency - in fact, I plan on trying the Z-1 CPAP as a travel machine. I will set it at my 90% pressure and I doubt I'll have any problems on a short term basis.
(Note that I used an S-8 Elite (single pressure CPAP) for several years before moving to the S-9 AutoSet a couple years back. I was doing fine on the S-8, it was just getting tired at over 15,000 hours. The pressure I had guestimated/titrated to, and that was working, is what the S-9 showed as my 95% pressure.
As it's to be a backup, there is no pressing need and you can likely wait until you can get a good deal on an auto if that's what you really want - especially if you are in an area with a large population and CL is fairly active (sounds like you are). That is if one of the more listed providers that deal in gently used machines can't fix you up. Had real good luck with them as well as my CL dealings.
I live in Phoenix, AZ. 4.5 million people squeezed into this valley :-). Actually, they aren't really squeezed. From Buckeye on the west to Apache Junction on the east is about 75 miles. Urban sprawl at its ugliest.
So I went ahead and picked up this single pressure unit - the person selling it was only 15 blocks from my house. It has 500 hours on it, and the person selling it wanted $50. Makes you go hmmm, as there are a half dozen or so going for up to $200. Got it home, found it had a bad power supply cable - machine would power cycle every five minutes. Didn't find this until I had worked with it for a while, and it only did it when you moved the power cord. Not sure if the person who sold it to me even knew this. So I replaced the power supply, and it is rock solid. Quiet as can be. And like I said my pressure is so low, and the differential so small, I can use a fixed pressure unit just fine.
Wow. $50 on a machine that had only been used for two months was pretty good deal for a backup. Even if you had to spend $50 on a new power supply.