"You...you realise that people get the "Experienced Mentor" badge just for posting a certain amount of posts, right? As in, they don't actually work for eBay."

There's a bit more to it than just posting a lot, and although Mentors don't work for ebay, they are expected to be an "advocate of ebay".  The following link details how you get to be a Mentor and what's expected of you:

https://community.ebay.co.uk/t5/Community-Notice-Board/Community-Mentor-Programme/ba-p/7612430