Those of us trying to get a rendez-vous at the prefecture to renew or apply for a titre de sejour know how difficult it has become. The site for the Herault prefecture is so saturated that it is virtually impossible for those with a titre to renew it. But foreigners who fear they may become illegal (like many Brits with Brexit looming) have been resorting to underhand black market schemes to get a slot.
Enterprising types have found that surfing all the avenues throws up rdv's which they reserve using fictitious names and email adresses. Then like all black marketeers they find desperate clients willing to pay between 50 and 110 euros for the time. Once the deal agreed they cancel the rdv and re-book in the correct name.
The prefecture has become aware of this and has ceased to post slots as they become free, but on a random basis. We cannot see how this will improve things but no doubt the crooks will suss it out.
Apart from being amoral, it is illegal.