Behind the specific problem of eBay prioritizing buyers over sellers, there's a more general problem of how marketplaces can determine who's right in a dispute. Other commenters have suggested taking a video while packing up the item and unpacking the return. However, that could be vulnerable to fraud as well; how can you prove that the package you film yourself opening is the package the other party actually sent?

Perhaps UPS/Fedex could act as a verification service. For a small fee, the UPS store employees could photograph the contents of the package before it gets mailed, check if the electronics turn on, write down the serial numbers, etc. and send a report to eBay.