Classified theme/plugins

HI

I am a new wordpress user and I am looking for a free/paid theme or plugin that can support placing ads for free but will charge buyers a fee for site access time.

e.g user can browse free advert, but if they want to contact they pay a 1 off fee after sign up for a limited time access to the full contact details of all ads.

I have had a good browse around and cant find anything suitable as of yet.

Many thanks