I've given a list of Traffic exchange sites which allow to post classifieds at free of cost or very few charge at the question "How can I find good sites for free or low-cost classified ads? " in the "ASK SC". You can get registered to some or all the sites in the list and monitor the performance for a week and mark the top performing one as the best and focus placing your ads and have a taste of success.
Go for it. Hope for the best.
