A used flash can be a good buy. The main risks are the same as with any used gear: wear, hidden damage, and unknown treatment.

Things to check:

• Battery compartment: look for corrosion.
• Moisture damage: signs of condensation or water exposure.
• Physical condition: dents, cracks, or signs it was dropped; inspect the hot shoe carefully.
• Zoom head: if the flash has motorized zoom, make sure it works properly.
• Diffuser/bounce parts: check any pop-out diffuser or moving parts for damage.
• Operation: test it in TTL and manual mode.
• Power levels: fire at full, medium, and low power to confirm it isn’t stuck at one output.
• Buttons/menus: make sure controls and settings respond normally.

There’s no special reason to avoid a used flash if it’s in excellent condition and functions correctly. Many photographers buy used flashes successfully. The biggest limitation with online marketplaces is that you may not be able to inspect or test the unit first, so buying locally or from a seller with a clear return policy is safer.