Allow members to send private messages only if the recipient is following them. Requires the BuddyPress Followers plugin.

This plugin only allows friends and site administrators to send private messages on your BuddyPress site.

CubePM is a private messaging system that integrates right into the frontend of your site.

Easily add Private Messaging for your users to your WordPress site with Cartpauj PM (Private Messages).

User can contact seller directly from product page. Option to set so that user can contact seller only after purchase.

Front End PM extension to integrate with Ultimate Member

This plugin allows the site admin to restrict who can send private messages or to enable the users to choose themselves.

Front End PM is a Private Messaging system and a secure contact form to your WordPress site.This is full functioning messaging system from front end.

This plugin is a Swiss Army Knife for messaging, It allows the site admin to place restrictions on public and private messages including general rules and quotas per role, message length and control over notification emails.

MDC Private Message is probably the best plugin to create an easy messaging system in WordPress.

WP Bulletin Board (WPBB) is an easy to use forum for your Wordpress site with Facebook and Twitter features.