Re: Buying a car in France, for commute to Geneva.
Post 5
I transfered my car from Germany to France, so if you pay attention to some points (e.g. the car shouldn't be older than 5 years), it's quite easy.
I recommend to buy a car in Germany because it's (much) cheaper than in France.
If I remember well, you need those documents:
- purchase contract (and I guess the car must have been in your possession for at least 6 months before you bring it to France otherwise you might have to pay the difference of TVA - but I'm not sure whether this is still the case)
- certificat d'homologation
- German Abmeldeschein (radiation des sceaux)
- Fahrzeugschein (carte grise)
The most tricky thing is to get the Abmeldeschein (I removed the plates from my car, send them to my parents, they handed them over to the according Authority and sent me the proof). This means, for some days, you won't be able to use the car.
When you have all documents, speak some French (or take a French speaking friend with you) and the car is less than 5 years old (otherwise you need a "Contrôle technique"), you have to go to: Trésor Public with the purchase contract to inform them about your import, then to the (Sous-)Préfecture (to get the French Carte grise in exchange for your German Fahrzeugschein + fee) - they issue also your plate number, then you go to any garagiste you want who will make the plates, and finally you get a car insurance.
Try to get a proof of your current car insurance stating how long you already have your driving license and whether you had accidents or not. I went to the French partner insurance of my German car insurance so that they accept the paper otherwise you will be considered as having no driving experience and consequently pay much higher premiums. 
I managed to do it all within half a day. Good luck!