Ask about buying used car in France

Im new comer in Paris , I find that the price of car in France is much higher than in United Kingdom. Some people say that i should buy car in Belgium and Portugal to get up to 20% cheaper than in France. Should i buy used car in Portugal or Belgium to use in Paris???

I just found seri 3- BMW car in Autoscout24 website in all Europe , whereas car price in all europe is so high.

I want to work for Uber because Uber driver is the best way to explore Paris.

Do you know which website is the best for find selling car online in France??

Thanks for your help