When evaluating used digital cameras, the most critical metric is the shutter count (actuations), which indicates the remaining lifespan of the mechanical parts. For professional DSLRs, a count under 50,000 is considered excellent. Additionally, inspect the sensor for dead pixels or scratches, check the lens mount for wear, and ensure the autofocus motor operates smoothly without grinding noises. For mirrorless systems, verify the EVF (Electronic Viewfinder) clarity and the integrity of the IBIS (In-Body Image Stabilization) system.