Some people will always find a way to cheat others no matter what the subject.
In this case, the scam is, once again, directed at foreigners, and involves the paid booking of appointments for Cartes de Séjour.
The scammer creates a web site, and claims, for a fee, to be able to book appointments and help you to queue jump a busy list, not just for carte de séjours but also for requests for naturalisation, changing driving licences etc
Whilst not actually illegal, this is a scam and the sites, which may often look quite official, only intend to cheat you out of your money, or gather your contact info for a variety of unhealthy reasons.
This is not to say that there are no honest services offering to help with applications – but do be careful to whom you hand over your credit card details, particularly if they are promising rapid appointments which don’t seem to be available to all.