Many sellers don't realise that they can add video into their listings. It isn't officially supported by eBay but it can be done. This is a guide on how to add your Youtube Videos to eBay listings. You can view various examples of how I have done it myself with some listings if you would like to have a look.
The way to do it is to upload your video to Youtube. Once uploaded you can view your video and choose to share it by clicking "Share" underneath your video.
Click on "Embed".
I recommend using a size that compliments the rest of the listing. If you have a narrow listing you want a smaller video size like the default option, but if your listing is wide, a small video may look out of place so may need increasing. If you do want a different size you can choose whatever size you like.
Make sure "Show suggested videos when the video finishes" is unchecked as you don't want your potential buyers to get distracted.
Make sure to have "Use old embed code" checked otherwise this will not work.
All that is left to do now is to add this code within your html listing.
I hope this helps!