Hello sillyship -
eBay does not accept Javascript in listings, so the embed code that is available from YouTube will encounter difficulty if you attempt to use that in a listing, but you will need to have the video available on YouTube to be able to post to eBay.
You can do this from the lister page in Auctiva while creating your listings, but will need to use a more appropriate version of the code you can get from YouTube. You get the code from the YouTube page where your video is. Beneath the video player click on 'Share' and then on 'Embed' and a you will see a piece of Javascript to embed the video. Copy this piece of code.
In order to make the code usable to post to eBay it will need to be converted to a format that eBay will accept. There are converters available online, but an easy one to use is available at:
http://www.isdntek.com/tagbot/YouTubeConverter.htm
This will present you with another piece of code that you can embed into your listing and then post to eBay.
When you are on the one page lister creating your listings, you will need to set the Item Description Editor to HTML mode by clicking on the 'Source' button in the upper left of the editor tools to add this code; please keep in mind that the video will appear in the item description wherever you place the code.
You will be able to check the placement in Preview to ensure that the video appears where you prefer in your listing.
- Craig