There is a Report button on every listing (on every posting too, come to think of it.)
When you Report something to eBay, they look into it. They may agree and pull the items or they may disagree.
You can also report some items to the manufacturer, especially if they are part of the VERO program. The manufacturer's complaints get faster action than those of a private citizen.
But this will not "warn" other buyers. Think about this. If a vigilante were wrong or being mischievous, a perfectly honest seller could have his business destroyed.
That's why these reports go through a third party.