eBay:  
The first and most powerful line of defense against tickets being sold for inflated prices on ebay is for individual users to flag the listing. Here's how you do it:

1. When viewing the listing, go to the bottom of the main description, where there is a banner suggesting other items you may like. To the right of that you'll see a link that says "Report this Item". Click it. 
2. You'll then be prompted to login with your eBay user name. Don't worry; you don't get called into the principal's office or anything!
3. Here are the selections you should make:
   Report Category: Prohibited and restricted items
   Reason For Report: Other prohibited or restricted items
   Detailed Reason: Event ticket resales
4. Click "Continue" and you're done! The more people report an item the more likely it is to be taken down.

StubHub:

Send an email directly to yourfeedbackmatters@stubhub.com containing the name of the event (2024 Burning Man Festival and 2024 Burning Man Festival Vehicle Pass), the dates of the event, and URLs for each listing you want to report. IMPORTANT: Include your contact number so they can reach you if they have further questions — they’re far more likely to take the complaint seriously if they can actually reach someone to respond. They want to help, so don’t abuse the StubHub folks, they’re not the ones who listed the tickets.

Craigslist:

Offer the buyer face value plus fees. If that doesn’t work, flag the post and it’ll be taken down … do it often enough and maybe the seller will be more willing to listen to reasonable offers.

Facebook Market Place:

Just as with Craiglist, offer the buyer face value plus fees. If that doesn't work, flag the post. Facebook might take it down, and if it is done often enough, the seller might be willing to change their mind.

Important to note: 

Unfortunately, most marked-up postings, regardless of which re-seller site is used, don't give us enough information to be able to pursue the buyer and cancel their tickets. Some helpful folks have opted to go through the process of "buying" the tickets in order to get the seller's info, then have reported it to us (with screenshots/supporting documentation), and then canceled their transaction. This is a bold move, and while we can't explicitly encourage you to do it it does have the benefits of 1) immediately getting the posting down and 2) collecting the seller's info so we can cancel their tickets. If you have any questions about this please don't hesitate to contact us at ticketsupport@burningman.org.

If you come across a posting that has some identifiable information about the seller, their name, order confirmation number, or (once physical tickets are in the world) a ticket number or QR code, email us and let us know so we can research and pursue it. Without that our only recourse is just to report items just like anyone else.

Remember: friends don't let friends buy marked-up tickets!