To insert tables into the item description of your eBay listing you need to use HTML or Markdown code. You can edit the description source directly in Editor Mode.
The conversion mode popup lets you choose between HTML (“Don’t convert”) and Markdown (“Convert Markdown to HTML”).
Markdown Tables
You can see an example on how to create tables in Markdown here
HTML Tables
This is an example of how to create tables in HTML. You can see how it looks like in here.