I think it's a good idea. There's definitely a market for having a central place to list things for sale in ONLY bitcoin. Craigslist isn't very usable for bitcoins right now because there's not enough people using bitcoins. For example, in my hometown in Oregon, there are no results, and in Portland Oregon, there's only one, and it's only an offer to buy bitcoins. Basically, it's pointless to try and browse craigslist for things for sale in bitcoin.
BUT, you need to reach critical mass. You need to have hundreds, if not thousands of items listed for sale, in order for the site to grow on its own. You need to promote the site until you reach those numbers, and you start to see steady growth without promotion.
I would suggest promotion campaigns, such as "I will pay you 0.05BTC for each legitimate ad you place in a separate category on btcpost.com". If you're not willing to put some incentive (or at least heavy advertising) into people posting on the site, it will not grow and it will not be successful.