You picked a hell of a day to start your site in a highly competitive market. Groupon launched their IPO today valued at $16.5 Billion and employ over 10k people. You also have Livingsocial and Google Offers and the almighty Google is struggling to get just 1% of the market.

Your either going to have to hire someone to make a custom script or pick a niche market and just concentrate on your local community. I honestly don't see the point in trying this without a big investment even for a niche site. It's almost as crazy as asking how do I build a search engine or an iPhone.