Are you using Internet Explorer as yout browser?
Recent updates to Internet Explorer (9, 10, 11) that are automatically installed by Microsoft has caused issues with the Sell Your Item form tabs for Standard and HTML. If you try using compatibility mode in Internet Explorer options, this seems to fix the problem.
When you set compatibility mode, it is for eBay only. You might need to set compatibility mode in more than one eBay page since eBay uses different URLs on different pages.
Here is a Microsoft web page describing how to set compatibility mode
http://windows.microsoft.com/en-ca/internet-explorer/use-compatibility-view#ie=ie-11
On the above web page, you can choose which version of Internet Explorer that you have (9, 10, 11).