I'd like to develop iOS app, which connects people offering some kind of service, with people looking for it.
Basically it'll be a free app with list of service providers and their contact info (backend on parse.com). If someone would like to be added to that list, then he will have to buy inapp purchase.
I'm wondering if this model is allowed by Apple? Apple won't reject my app?
As proposed your model does not seem to violate any guidelines, IMHO. However, you must be 'selling' only the 'advertising' for the service providers. And you are, since the service provider is buying the IAP, not the service receiver. You cannot also allow the service receiver to obtain any service from the service provider through an IAP purchase. That would violate guideline:
11.3 Apps using IAP to purchase physical goods or goods and services used outside of the App will be rejected
Yes, purchasing inapp will allow user to advertise in my app (add entry in my DB).
Have you been successful?