It depends on the customs regulations of your country (France, presumably). I bought my first digital SLR (EOS 300D) during a trip to the USA when it first came out, as well as a bunch of lenses and accessories (from a mixture of Adorama, B&H and a local camera shop in Vermont). I brought them back home to the UK (where I lived at the time), but first made sure that I had thrown away all of the packaging. At that point, the camera equipment became classified as 'personal effects', like anything else you'd take on a trip abroad (eg, underwear, an iPod, etc).
It's the same here in New Zealand. Once it becomes a personal effect, and not a new product still in its packaging, you're okay and don't need to pay import duties. The packaging is significant because by keeping it, it signifies that: a) the product is brand new, and b) you might be importing it to resell - which means you'll get taxed.
Most people take cameras on their overseas trips, and a used camera is extremely unlikely to get any special attention by customs officers.
I do buy a lot of items from other countries (I buy my business shirts from a shop in Paris and have them shipped to New Zealand, for example), so I'm reasonably well-versed in my own country's policies, but it would pay for you to check your own.