The best description I have heard/read ... is ..

2nd hand CNC's are like 2nd hand haulage trucks ...
They were bough new, and bought for a purpose
Once they no longer are fit for that purpose, they enter the second hand market !

Yes, they are cheaper, and still viable to those that have the knowledge to 'tinker' with the machine once a week/day/etc. to set it back to the best it can be.

but usually they have been worked hard, and you can not tell how it will work for you until you try it !

As others have said ... let us know more about what you want to do, and you will get more precise answers :)

Good luck !