The LEx platform has a built-in messaging system that allows you to communicate directly with other members of the community. To message a buyer/seller about a specific transaction you'll need to get to the order details page (click My Account > My Sales/My Purchases).
To send a message, click View...
You'll find the Send Message button in the buyer/seller information box on the right...
To message a seller before making a purchase, click the orange Ask the seller a question button from the listing details page.