Limit amount of entries a user can create based on their user group
I think a custom plugin is going to be the way to go with this.  Your template code would look something like:

{% set entries = craft.entries.section(‘listing’).authorId(user.id) %}

{% if entries.count &gt;= craft.myPlugin.getEntryLimitForUser(user.id) %}
    No more entries for you!
{% else %}
    More entries for you!
{% endif %}


Your plugin would also hook onto the onSaveEntry event and keep an incrementing entry save counter for each user you're tracking.