Buy with a guarantee. Test soon after purchase.
It it has a bashed or stuck filter walk away.
Stop it down, dop and decouple, check the iris shape and blades in the light. Regular shape only. No signs of oil.
Mount it again. Press dop again on and off, observe the snappiness.
Take a body with you, or an sd cf card at least. Its a camera shop trick not to have memory in used cams, saves you looking to closely.
Check infinity focus, turn the mf ring manually, turn the zoom ring, any stickiness walk away.
Look through the lens from either side with light behind it. Ignore fine dust. Beware anything with a distinct shape. Any sign or smell of mould, walk away and wash your hands before touching your own kit. Burn your clothes.