No, I think it is one of the least effective types of link building that you could do. Focus on getting one-way links from high quality authoritative websites in the same niche as your website is in. The quality of the links is far more important than the quantity, and this is especially true after the Penguin update.
not too much, they usually remove the links after few weeks when the ads are expired. you better look for a permanent links from high authority sites.
Free Classified is great method to get the traffic even you can sell products of your sites...Really its very useful..
Classified Ad Posting is best seo technique to get back links & boost traffic. Always try to do work in high page rank sites.