The default listing details template design contains a "contact seller" link within it. Clicking that link will send the client into the contact seller page to ask the seller a question. You have the option of placing that "contact seller" form within the listing details page directly if you wish. To do so you only need to insert the following tag within the listing details templates. The default listing details templates are:

/main_page/listing_classified.tpl
/main_page/listing_auction.tpl

The tag to place the contact seller form within those templates is:

{listing tag='contact_seller_form'}