Virtually all agencies will say you're self-employed (they don't want the legal responsibilities that come with having you as an employee). I still think this is dubious (an escort-agency working relationship will rarely pass most of the usual tests used to distinguish between employed / self-employed status, like being able to send someone else to do the work in your place) but there is an interesting case saying otherwise in the even more extreme context of a brothel.
But here, yes absolutely: scam, scam, scam.
The 'we will only show your ad to people who jump through hoops' is a particularly annoying scam: not only are they an expensive ad site, they're an incredibly crap place to advertise, but they may well do enough to avoid being successfully sued by victims.
There was a fascinating report on Radio 4's Today programme this week about similar problems with scam acting/modelling 'agencies', complete with a campaign to ban upfront fees entirely.
Your own site would cost you about ?3 a year to register and it's quite possible to do your own without paying someone fifty times that much to do it for you.