I'd literally need someone for the financial clarification. Any volunteers?
I mean "how legal it is to earn money from such a website"...
Making money from selling advertisements on the internet is not illegal, but that is a naive view and is not the real issue.
In the Netherlands prostitution is a somewhat regulated profession, but prostitution by someone older than 18 years old is not illegal. However, there are additional conditions related to your nationality/citizenship (European Union or not); whether you can work as an employee in a brothel or not; if you are not an employee you have to register as a self-employed worker; or you can have an "in-between" position (neither employee nor self-employed) if you comply with the rules of the "voorwaardenpakket". See for more details:
https://www.rijksoverheid.nl/binari...and/werken-in-de-prostitutie-in-nederland.pdf
Let's call people who are not in one of the above three regulated categories "unregulated prostitutes".
Is working as an unregulated prostitute illegal? In a broad sense, yes, but it is more of an administrative illegality, rather than a criminal illegality. It is more or less the same as people having a second job for which they don't pay taxes or social security and which is traded on grey or black markets. Think of your Polish cleaning lady or some handyman who does some small repairs in the weekend or in the evening and gets paid cash.
Is selling advertisements for unregulated prostitution illegal? Again, in a broad sense, it is. But that would then be as an accessory to assisting unregulated activities. Coincidentally that activity is prostitution, but that is entirely irrelevant. No unregulated prostitute will be fined or jailed for being a prostitute. She will get an administrative fine for failing to register herself as a self-employed worker, not paying taxes and not paying social security dues. I am not aware whether being an accessory to such matters is ever prosecuted, or if it is even possible. If you want to know, get some legal opinion from a Dutch lawyer.
A more serious issue however, is that of "pimping". A pimp is anyone who benefits from someone else's prostitution, whether regulated or unregulated. Only properly licensed brothels can escape this situation. Renting "work" rooms for prostitutes is a bit of a greyish area, but probably regulated at the city level. Getting money from publishing advertisements for prostitution might be in that same grey area, but I can remember no recent court cases in that respect. At worst, it is something the Police does not have the urgency to spend any time or resources on. At best, it is not considered pimping.