When purchasing used tractor tyres, the most critical factor is the Remaining Tread Depth, usually measured in millimeters or as a percentage (e.g., 70% tread remaining). You must also verify the Ply Rating (PR), which indicates the load-carrying capacity and casing strength. Ensure the Bead Condition is pristine, as any damage here prevents a proper seal on the rim. Additionally, check for Sidewall Integrity to ensure there are no deep cuts or 'weather checking' (cracking from UV exposure) that could lead to blowouts under high torque.