Assuming you're building something you want around long-term, I'd recommend starting within a certain metro area, like Miami.
Focus all your efforts on building ads there, in fact, reach out to people and offer to post ads for them. Once you have a respectable position in Miami, the rest of Florida will follow, then the southeast, etc.
I've got several thousand real ads that I use on classified sites, PM me if you'd like to upload my feed to your site.
Good luck!