Hi Pinks,

Craiglist is a free classified site with high traffic. It is good source of driving in links and traffic. Although the listings have a certain time-span, it provides your website with a good bit of traffic with a single ad. It is good for indexing as the Search engineslooks for optimized ads in the higher ranking directories It is very important to place your ad to right catagory and without unique titles. Learn to place and optimize your ads. Thats it. 

Do not worry about Rankings because its sales that you need at the end of the day.

Hope this helps. Thanks