They explain that in the article (the second sentence) - as well as the regular autotrader domains, they host sites for specific dealerships too, which are presumably just a rebadged version of their main site.
Almost. The dealer websites are templated CMS's which allow dealerships to add and manage stock whilst seeing the current price around the local area for the specific vehicle that is being listed (based on condition etc).
These sites and the stock are then entered into the main database which is indexed and presented to users via the search.
This has changed and improved since.
NB: I used to work for Autotrader back in 2011 before they moved to their current home.