Private messages (PM's) can only be sent between users once an offer has been accepted. PM's are used to facilitate the safe exchange of personal details between users.
If a buyer unofficially makes an offer in the comments, they will still need to make an actual offer using the 'place offer' button, and have the seller accept it, before either person can send a private message. Buyers have the option to add a condition to an offer, such as 'pending inspection', so that private information can still be exchanged in order to view an item before fully committing to a purchase.
For services, swap, free, wanted and pet adoption ads, because there are no offers, private messaging will still be permitted when the exchange has been agreed within comments. Cars, jobs and property ads also do not require an accepted offer in order to send a private message.
How to send a private message:
To send a private message, go to the user's profile page and click "Send Private Message", or just click on "Send Private Message" on a buyer's comment on one of your own ads.
You can view your private messages in your "My Profile" page.