Writing an effective classified ad is one of the most exacting forms of copywriting. With twenty words or less you have to say something that will cause potential customers to click on a link for more information.
Classifieds are lead generators.
You never convince someone to spend money in an ad. You will need to automate your followup and personalize the replies to those that reply to your classifieds. You will also need to be able to get a tracking report from your responder system. Any other way is just too time consumming. Your first message shouldn't be too long but should give them the desire to look at your web site.