Based on c5owners.com's buying guide and owners' experience.
Have the motor and electrics been tested?
A C5 described as “untested” usually just has a dead original battery and nothing to connect a modern battery to. Many run fine once powered up. But buying untested is a gamble on the pod's ULA chip and the control box.
Keys. New C5s came with two keys: one for the master security (power) switch and one for the boot. Boot locks can be replaced; 3D-printed boot locks exist.
Front wheel cover. These are rare and expensive to replace.
Rear wheel discs and centre caps. Check both sides. Reproduction centre caps exist.
Is the original key switch and battery top still fitted?
If yes, you can keep the original arrangement with an Oldham-pattern battery (types 015 or 038), or use a 3D-printed adapter that lets the original key switch clamp onto any battery.
If not, look at how it has been rewired. A common conversion is a 12 V relay switched by the light switch.
Accessories add value: mirrors, indicators, High-Vis Mast, mud flaps, side panels, horn, tonneau, and boxed items especially. See Accessories.
Boxed or unused examples with a sealed handbook, charger and tool kit fetch premium prices.
Check the condition of:
Plastics: cracks around the boot hinge points, rear location frame, pod mount and mirror holes.
Chassis: rust on the Y-frame and the black rear support frame.
Gearbox: noise, and whether it has a metal insert.
Belt: slip or clicking.
Brakes: the rear grabbing, or the front rim being heat-damaged.
Tyres: original tyres are usually cracked.
Motor end-cap colour: brown is early and brittle; pale is later.
c5owners.com: “What to Look for When Buying a Sinclair C5”; “Restoring a Sinclair C5 on a Budget”; forum threads on LiFePO4 batteries, tyres and gearbox inserts.
Sinclair C5 Owner's Handbook (1985); Hoover Service Engineer Training Notes (1984); The Unofficial Sinclair C5 Service Manual.