Generally speaking, I've tried to stay away from messing with products or services that require a lot of marketing, and go with ones that tend to market themselves. General ideas (such as classifieds) have big potential, but require the most work to get sustainable traction.
With that said, building the product is half the battle. Put it up by yourself. The biggest question you need answered right now is what do random visitors to your service think of the overall concept? Any flavor of classifieds websites will have some kind of critical mass, but that wont stop a few visitors from making an attempt at using it. If you get some positive feedback, or even negative, use it. If it's positive, it will go a long way in finding a valuable partner. It will give you hard data to share that will start to pave a path for you.
The problem with splitting a pie that hasn't been baked is that nobody knows what the flavor will be. You may be a great baker but the consumers are the ones who will inevitably decide your offering (Apple).
Once you get a basic data set from those few visitors you were able to convince to at least give your service a glance, now it's time to grow your inventory. The best things you can do as a lean start-up is find some mutual business relationships. Can car dealers post some of their inventory on your site? If so, they would love to do it and it gives you a little bit of content. There are many other similar circumstances you can find yourself in. I find it most effective when you can get other businesses to sell your product for you because it enhances their own.
To get back to the root answer here, my suggestion is do a solo soft launch. Gather usage data from early visitors, and use it to grow your inventory and perhaps more importantly, find a valuable partner.