Maybe if you created a type of aggregator where you listed cars on various car websites. When a user clicked on the listing, they are brought to the original thrid party site where the car is listed.
You would of course have to seek permission from the owners of the other third party sites to do this. Some websites may have a web service where they can feed the listings to your site. On the other hand you could make a scraper (or data miner) to pull the content from their site, again with this idea you will have to discuss the matter with the owner of the site as you will need to agree on bandwidth usage, storage policy etc...
You could make a few quid from advertising with the likes of google adsense or affiliate programs like you see on this site. And you could of course list your own cars on the site.
An example of this type of idea is
http://www.jobrapido.ie - it's where I found the job I'm currently in!
I was involved with a property website before where one of our competitors was using the contact number for property owners as displayed on our property listings to contact the property owner directly and ask them to list on their site. We only found out about this when one of our customers reported it to us. Now that was a very sneaky (blackhat) way of doing it. When the owner of the website I worked on confronted the competitior about it their attitude was something along the lines of
"Tough! You posted the contact details freely."
The owner threatened the competitor with legal action but I left soon after and never found out the outcome.
Bottom line is you shouldn't cheat at something like this. It is morally wrong! You should always ask for permission and come to an agreement that will help both your website and that of the third party.
