Grinders finish parts to tight tolerance and fine surface finish using an abrasive wheel — surface, cylindrical, ID/OD, and centerless types. Precision and rigidity are everything, so wear and rebuild history drive value heavily. Well-kept precision grinders from top builders hold value.
See every Grinding 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.