What are the biggest reputable online camera stores in France that offer used equipment for sale? Is there anything similar to keh.com either in France or in nearby countries (with affordable, customs-free shipping to France)?
Both have physicals stores and their used products come with a 6 month warranty (at least for bodys and lenses). I bought one used lense from "Objectif Bastille" 18 month ago and have still no problem with it. A friend bought his body from Phox (about 2 years ago) and is still happy with it.
If you are looking for non-professional website (and speak French), you can try Le Bon Coin (www.leboncoin.fr/), it's a website similar to craigslist where each person can add a "for sale" post. As usual with such website, you should take precautions when buying.
I recommend eBay France and PriceMinister . On Priceminister, people often have no idea what they're selling so you can find interesting prices. Both will offer you buyer protection and as far as eBay is concerned, often low prices. You can also stay tuned to your local Craigslist and Le Bon Coin which was already mentioned.