There's a lot of wasted whitespace to the right of your logo. You could put login an search functions in there and free up valuable real estate for extra pricing tiers.
Ad model is good with free option and tiered plans and prices listed. However, if I'm looking to advertise I want stats on visitors, conversions and other demographic info. Also you could put dummy company info and logo into the Summary and Detail view examples as opposed to blank stuff.
You're going to need to up the SEO a bit especially as there is a lot of competition out there. Seems like there's a new
classified ads site every week or two.