Reply to the ad, it will bump to the top. Most sellers reply with "BTT", which stands for bump to top. Make sure you do not reply whatsoever until 72 hours have passed or your ad is on page 4 of the section you are selling in, even if someone asks you a question. Email or IM the answer to them, or edit your original ad. Otherwise, answer and bump after 72hours/page4.
Subscribe sends an email to your address telling you when anyone posts in the thread you subscribed to. Unsubscribe is just the opposite, it turns the emails off.