Please use the Private Message system or members "walls" to communicate with them. Especially in the For Sale and Wanted sections. These features take place of whispers.
To do this, click on a members name. From here you can post on their profile page (this is visible to anyone) or you can click on the link to the left of the page to send a private message.
Additionally, you can set your preferences to email you anytime someone sends you a private message or posts to your profile.