My Classified/Reverse Auction Website

My website <snip> is a classified ad/reverse auction platform that allows users to post free want ads and receive competing offers from sellers. I have initiated a GoogleAdwords campaign on a modest daily budget and have had 125 unique visitors in 4 days. This is good but only 2 of them have registered and created profiles. This is discouraging to be because it is free to register and less than 2% are doing so. Adwords is not going to be cost effective for me to continue using. I have facebook, twitter and a blogger account. I need to target an audience who will register for the service I provide. Any ideas?