The link below is to an eBay help page to describe how to add a video to listings.
You can only add one video per listing, but you can use the same video on multiple listings - you just need to upload it once. Upload a video using the new listing tool by selecting Add video from the Photos & Video section: - The video should be .mp4 or .mov with a 150MB limit, or about one minute long eventually https://www.ebay.com/help/selling/listings/creating-managing-listings/add-video-to-listing?id=5272
