From the Marketplace home page (to get there, click Marketplace on the left side of your Home page) or the search results page (on the Marketplace home page, click Search), click the name of the listing you want to respond to. Facebook displays the full ad, complete with pictures (if the person who placed it supplied pictures, that is).

Do a quick reality check. See how many times this ad has been viewed (the number appears at the bottom of the page), which gives you an idea of whether the item is still available. Then click the picture of the person who listed the ad; this sends you to her profile, which you can browse to get a feel for whether you want to conduct business with her.

Note:

If you're the kind of person who likes to get a little feedback from friends before making a big investment, you might want to click the "Is this a good deal?" link (or one of the other Ask Your Friends questions that appears on the left side of any Marketplace listing). Doing so blasts your query—along with a link to the ad you're considering—to your friends' News Feeds and leaves an automatic comment based on your choice (such as "Is this a good deal?") in the listing's comment section.

To answer the ad, click the Respond button. Then, in the Respond box that appears, fill out the Email and Message fields. (Your email won't be passed along to the seller; it's for Facebook's records.) Unless you have a good reason not to—the deal seems way too good to be true, say, or the seller's profile picture reminds you of a Wanted poster you just saw—turn on the checkbox next to "Allow marketplace to share my Profile name and image with the seller" and then click Submit.