IMO, you found success, If you and I were working that idea, I'd be exploring it more!
This is a common user story:
"My car is busted, miles are high, I hate having to put more cash into this pig. Maybe I'll take the cash to fix and use it to trade it"
Is this every user's story? No. But, it's a high value user!
The test:
"Will adding inventory onto the page create a productive visit for this user's story?"
DISC? (Does It Sell Cars?)
--Separate and bucketize shoppers that click on cars from service page.
--Compare T.O.S. vs regular shoppers and vs service only shoppers
--Compare repeat visit behavior [very important]
--Lastly, Compare contact info from service appointments vs sales.