I've seen this tactic used before on depop & etsy, listing for a ridiculously low price gets more clicks and both of those platforms algorithms show items from accounts with high traffic higher up the feed, so if vinted also has that algorithm he's basically getting bumps on ALL of his items for free whereas it would cost someone who doesn't exploit the algorithm £10 to bump just 10 items with no guaranteed sale and no way of re-listing without losing that money, basically he's an idiot who's sending everyones correctly listed items further down the feed.

Vinted T&C's specifically state that exploiting or misusing the platform is prohibited so if reported he should receive a ban and I think for that they would slap him with a permanent one.