Inspect each shell for cracks, especially around the lugs (the hardware mounting points), since a cracked shell affects both tuning stability and resale value more than most other issues. Check that bearing edges (the rim where the head sits) are even and undamaged, since a chipped or uneven edge causes tuning problems no amount of adjustment can fully fix. Test all hardware (stands, pedals, hi-hat clutch) for smooth movement and secure tightening, and check drumhead condition for dents, tears or excessive wear, since heads are consumable and factor into your offer if they need replacing. A kit that's complete with all original hardware is worth more than the sum of individually sourced replacement parts would cost.