To stop your css links being stripped out you can create a product attribute and add them to the description that way.
Make a product attribute in Magento called "ebay-css" that is a text field or area.
Add the new attribute to your attribute sets
Go to a product admin page and in the field for the attribute "ebay-css" enter the css link as it would normally be in the html head, like <link href="https://mydomain.co.uk/ebay-stylesheet.css" rel="stylesheet" type="text/css">
In the M2E description editor, use the "Insert Customs" button to insert the "ebay-css" attribute at the top
Save the description policy and revise the product. It should now be linking to the style sheet nicely.