Since early 2017, eBay have prohibited the use of JavaScript, Flash, plug-ins, and form actions on eBay listings, as stated in this documentation article on eBay.
You can check whether your eBay HTML template is mobile-friendly by using one of a number of websites, for example: http://www.i-ways.net/mobile-friendly/en-uk/
For more information please refer to the post regarding these changes on our blog.