The CNC BP’s are usually set aside when some part of the controller goes bad. Usually by the time the machine is up for sale, this is a moot point, as the controller is obsolete anyway. In general the Servoes and encoders are extremely durable and can be reused. Change the brushes on the older servos every decade or so, and they keep on running. Updating the controllers can be done by piecing it together for a couple grand, or as a Complete set through Centroid for 3-5 grand.
The factory CNC BP machines have very large, fat servos with heat sink fins on them. That is the sign of an OEM setup. The older conversations have the thinner but longer servoes...these are brushed DC motors, and easily adapted to any controller you want to. The latest servoes have transitioned to stepper motors, which are thinner and shorter, in general.
The “fear” in looking at a CNC machine is, since the controller is likely inop, you can’t run the stops like a manual mill to check for wear. You could, but it would be a day’s job, and most sellers would frown on taking a day to take the servoes off the machine to check. Instead, you have to go by the condition of the visible ways, And general condition to indicate the care the machine has received. Most later BP’s have chromed Y ways that hold up to a lot of use. They will also have the zero lash ball nuts.
You can always manual mill with a CNC machine...but you can’t CNC with a manual. Once you’ve seen the capability of a CNC machine you will realize it’s worth a bit of risk getting into it. It is amazing watching 3 axis moving at the same time to perfect points. In my case, the mill came with an old HAAS rotary table In the deal...so 4 axis in simultaneous motion. It’s a whole new dimension in Milling capability.
Anyway...just throwing is out are an option. I bought mine thinking I would convert it back to a manual...but I was too dumb to know what a great deal it was until I studied up to figure our what I really got. Shear dumb luck! A lot better deal than I got on my lathe...but that’s another story.