Due to eBay’s listing policies you can’t use the code provided by YouTube or Vimeo.
What you can do is to insert a link to the video or use this online tool to create video code that you can insert in your listing (in GarageSale’s Editor mode):
http://www.isdntek.com/tagbot/YouTubeConverter.htm
It only supports YouTube, it seems.
Regards, Kristian