In eBay’s most recent seller update it was announced that in the coming months eBay will be switching to the HTTPS protocol to keep inline with industry security standards. Although it’s not clear when this update will be enforced, it will affect custom listing templates that include elements hosted on non-secure links (HTTP).

This is exactly what eBay said:

New requirements for secure images, listing templates, and functionality in item descriptions

To align with new industry security standards, eBay will begin exclusively using the HTTPS communications protocol in the coming months to send and receive information between your desktop browser or mobile device and our site. We have used this protocol for many years for transmission of most sensitive information and are expanding its use. Item descriptions that contain externally-hosted cascading style sheets (CSS), images, or HTML5 video may be affected. To ensure that your item descriptions and images display properly, ask your hosting provider or third-party partner to support HTTPS and update listing templates or descriptions accordingly. For optimized display on both web and mobile, we recommend that you upload your images to eBay.

In light of this update all Finest Design eBay templates have been updated. All hosted stylesheets or image files are now pointing to HTTPS links, with a valid SSL certificate residing on the server.

You can read about this update and what else eBay has planned in the summer 2017 update here:
http://pages.ebay.com/seller-center/seller-updates/2017summer/listing-policy-updates.html