Unfortunately no, the bonus écologique is not available when you buy a second-hand car; it is only for new cars, with CO2 emissions of 0 to 20g/km. In this case it can give up to €6,000 off the price, limited to 27% of the full cost.
However there is some good news – as of this year those buying a non-polluting car, whether second-hand or new, may be eligible for Prime à la conversion, a sum of money for those giving up an old, more polluting car for a greener one.
In the case of buying a second-hand car, either electric or a conventional one producing less than 130g/km of CO2 and eligible for a Crit’air rating of 1 or 2, you may obtain €1,000 if you are in a household paying income tax or €2,000 if you do not pay income tax (due to a lower income). Note that the technical term for a non-electric car in French is now voiture thermique (ones using a mix of the two technologies are known as voiture hybride).
The old car should be (for a taxable household) a diesel car registered before 2001 or petrol one registered before 1997, or (non-taxable) respectively ones registered in 2006 and 1997.
The old car needs to be left with an accredited centre for VHU (véhicules hors usage), which will recycle it.