Can the classified ads be setup to charge users to post an ad?

No, MemberGate classified ads are setup so non-members and members can see the ads, but only members are able to post an ad.

This offers incentives for non-members to join the site - if they want to post an ad, then they have to become a paying member.

Technically members actually do pay a fee in order to post an ad - in the form of the dues they pay to be a member.