You must make an official offer on an item to be eligible to rate the seller. Do this by clicking the "Make Offer" link on the item's listing page.
Assuming you do make an offer through the market system, you may rate the seller but only if you actually buy the item or if the sale falls through because of action or inaction on the part of the seller.
To rate the seller on a classified ad, simply click the icon adjacent to the item on the Buy or Bid tab of your My Market area. You can also click the "Ratings" button in the Marketplace navigation bar.