The contact information for each seller is available on corresponding pages.
You can ask a question about your purchased items or items you are interested in.
If you check seller information at the top side of the page, or scroll down the item page to the bottom, you can see "Question" or "Inquire to the seller" under Q&A board.
And if you click the button, it will redirect you to the page where you post the question to the seller.
History of your inquiries can be viewed at my inquiriy page in My Gmarket.