I can buy a DS1054Z scope locally on craigslist - what would be a good value for a used model? Depending on included probes etc...
I'd say $250 for a
really nice one.
How can i best test it? I do have a low power digital signal generator available...
Just connect all four channels to the nearest mains socket, make sure the sine wave is clearly visible on each one.
No, wait, you're supposed to connect the probes to the calibration signal and make sure it's 1kHz and approx 3V.
You have to ask yourself...Do I really know how they treated it?
Yep. Worry much less about the 'scope and a lot more about the previous owner. Are they the sort who used it with Arduinos or are they the sort who'd do the "Mains Socket Test" the same minute they unboxed it?
Check if all the little spring things and accessories are present and correct, not because they're valuable but because they show that the owner cared.
Original packaging is also a sure sign that the owner is the sort of person you should be buying from.
Car analogy: Look at the paperwork and service history
before you start the engine and take it for a test drive.