Beyond make and model, the biggest value levers are: spindle hours and condition; control generation (a modern control lifts value, a dated one caps it); included tooling and options (spindle upgrades, 4th/5th axis, probing, bar feeders, chip conveyors); accuracy and whether it has linear scales; and simple market demand for that machine class right now.
Documentation, a running-under-power demo and clean way covers all help a sale. See a transparent, data-backed value for your exact machine at The Machine Blue Book.
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