I noticed that many of the vehicle images didn't load for me. It looks like they're hosted on whatever site they're pulled from. It would be a good idea to save thumbnails of the images that you find so you can serve them locally and include them for every listing
Since you're scraping car data too, it might be a good idea to get some basic descriptions for each type of vehicle. You might be able to opt into the Kelley Blue Book Developers program or similar:
It would also be good to provide at least some snippets of the descriptions of the vehicles that you're listing. More textual content would give Google something extra to dig into and would keep users from leaving the site and just browsing ebay or Craigslist themselves as often
It would be a bigger effort, but having basically a page for each listing locally would be interesting. You could show the original text and posts from the original source. Make the source clear, along with the last time you retrieved the information from it. Then if the original listing gets removed, you could keep it around. Show that it was unlisted or sold, but have a banner at the top that says something like "This listing is no longer available. Click here to look for available [make] [model]s". It could help people to find why one vehicle was cheaper than another (condition, location, etc)
I'll talk with the team about richer data as it furthers our SEO goals. I think the idea here would be that better engagement will improve our ranking? Is that correct?
The tricky part is getting impressions and clicks but this could help.
Absolutely. Rather than performing optimizations for SEO, it would be better at this point to add more to the site. Once that's done, post about it in different places like on Reddit. You should start to see more traffic from both web crawlers and people alike