What is the CO₂ value?

Use the combined NEDC value (the older test, for cars first registered up to 2019; German: “NEFZ”). It is on the COC, the Certificate of Conformity from the manufacturer (usually item 49.1): ask the seller for a photo. In the listing only if it says “NEFZ”. Up to first registration 08/2018 it is also in field V.7 of the German registration certificate (“Zulassungsbescheinigung Teil I”).