The spindle holds and turns the cutting tool; its taper (CAT/BT 40, 50, HSK, etc.), power, torque, and top speed define what a machine can cut. Spindle condition is core to a machine's health — a rebuilt or worn spindle materially changes value. The taper also dictates which tooling transfers with a sale.
See every Turning Centers & Lathes in the reference library, by make and model.
How this affects buying and selling: read The Gauge, or value any machine free at The Machine Blue Book.