Le site nationalexpress.com est-il sûr ?

A good site, and a good service. Coach information and ticket costs, with the facility to purchase online and print your ticket out. Especially for middle-distance journeys, my experience is that their fare often works out to be slightly cheaper than on the corresponding train journey, and journey time is usually comparable too. Over long distances, their fares are consistently far cheaper than train fares, but journey times do tend to be longer - for example, Liverpool to London takes just over 2 hours by train, but just over 5 by coach. Caveat emptor - as with all things, do your research before you buy.