If you are using the eBay app, you should be able to scroll down the main page and find your garage, and from there use the menu to delete a vehicle from your garage. At least that is how it works in the Android version of the eBay app for me.
Otherwise, try using a browser, going to ebay.com and requesting the desktop site version of the page. Then go to eBay Motors and select "Find parts for my vehicle" in the upper left corner of the page. That should take you here:
https://www.ebay.com/g/mygarage
From there you should be able to delete vehicles from your garage.
