Not all categories allow multi-listing. There is a link on eBay that tells you what categories do allow it, but I can tell you it is not up-to-date. I know one category that does allow it is craft stamps. When you start a listing for craft stamps, one of the buttons to click on the listing is "create variations". You can view this yourself by creating a listing for craft stamps, then delete the listing, just so you can see what it looks like. Then when you go to create a multi-variation listing in your category, you know where it is and what is looks like. When you do this, start a new listing, do not use sell similar. Here is a pic of what the create variations looks like. If your category does not have the create variations, then you can't do it for your category.
