I asked one nicely to stop spamming me and they responded with something obnoxious as if they were genuinely trying to help me. I reminded him his actions were against ebay policy and I would report them. I doubt it mattered.
I stopped bidding on cars now, screw ebay.
Edit: worth noting, you don't need to win the car. Simply the act of bidding discloses your phone number.
Someone needs to take these lemonade-stand sellers down a notch or four. Were ebay to extend the feedback mechanism to pre-bidding / pre-purchase queries, the participants would be forced to clean up their act. It seems that ebay is not incentivized (enough) to do this, and so the situation persists.
eBay practically permanently bans everyone involved if you dare take conversation offsite on normal items.
Interestingly under UK law, business sellers must display contact details (address and phone) on listings. eBay seems to do its best to hide this info at the bottom of the page inside a js pop out.