What kind of cpap was it, and did the guy show you it was working before you paid him? I ask because those machines have settings that need to match your prescription, and a random one off the street could be blowing at the wrong pressure all night. Even if you slept good, the wrong setting can mess with you over time in ways you might not notice for weeks. Did he give you the humidifier part too, or just the base unit?

Paid for itself in 6 weeks" is the part that gets me. I saw a thread on a baking forum where a guy did the math on his stand mixer and it took him almost two years to break even, mostly because he only ran it a few hours a week. Six weeks means you're cranking through dough fast enough that the thing isn't sitting in a corner collecting flour dust. Used gear is where it's at too, commercial stuff is built to run all day so even a beat up one has more life in it than the shiny home models. People sleep on the used market and then wonder why their setup costs a fortune.