Modern CNC machines typically hold positioning accuracy in the range of a few tenths of a thousandth to a few thousandths of an inch (single-digit microns to tens of microns), with repeatability often tighter. Actual accuracy depends on the machine's build and condition, thermal stability, the control, tooling and how the job is set up — a worn machine holds far less than its original spec.
Wear and condition directly affect what a used machine can still hold, and therefore its value. See probing and what affects used machine value, and check any machine at The Machine Blue Book.
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