The good news is that you're dealing with high priced items, so you can charge higher rates. Your number one selling point is going to be whether or not being listed in your featured section offers a better chance of generating a sale.

You'll want to track clicks for featured vs non-featured items. Also, you'll want to follow-up by visiting the forums for your featured items, and then compare those to the average non-featured item. Are the items you're featuring ending in a sale more often, or faster? If someone is selling a $50k car, and you can prove they have a 20% higher chance of selling by featuring on your site, then paying $50 to get featured is a deal. However, I don't know if you have the current traffic numbers to have that sort of benefit.

You can try the above before you have a single paying customer. Choose 4 products at random, feature them, watch the results and increase in clicks.

When it comes to AdSense, don't expect to get rich. I'd say your site would pull in $3-10/day.

For an ad manager, I do a 50/50 split for direct advertising campaigns. The number is high because the individual managing my advertisements previously ran one of the largest companies in my industry, and they regularly meet with the large billion dollar companies that I would struggle to get time with. An equivalent for you would be someone that has great connections at Ford, GM, Chrysler, etc, so when Ford releases a new car and is running an advertising campaign, the related category on your site is a part of that advertising deal. How you find that person, I don't know. In my case, they contacted me because of my user base. Figure out the CPM rates you can earn on your own, then you can pay someone commission for anything higher. For example, you might earn $0.25CPM through ad networks, and then you can sell direct advertising or $2.00CPM, or find someone to sell it for you.

It's a really tough business, I can't stress this enough. AdSense is the best ad network for general fill, so give them a test first. Try the affiliate offers through CJ as well, they might work better in your industry than mine. For a while I was earning $20k a year through CJ on the side, but it went downhill in my business.

You might also want to try accepting posts on your site. Allow users to upload a photo, title, description, and contact information, then mix those with the results you're scraping. Over time, you could possibly phase out the scraped content and focus on user submitted content, and adding additional categories. Then you'll have a little more control over the content, users, and charging fees. A site with original content is going to be more valuable than a site aggregating content from other sources. However, aggregating content is a smart method of starting such a site, so you might be in a good place to make that transition. Definitely consider this option, it might help your site grow into something much larger.

Edit: One last point. Visit your competitors, or other classified sites. They've been down this road before, so see what ad networks they're using, what ad sizes and placements, what offers they're pushing, and how they're charging users, etc. They've likely spent years trying to find the perfect solution for them, so use their knowledge as a starting point.