I think this is a feature of Windows 10's 'Smartscreen' feature, which I noticed was installed in the recent v1607 (Anniversary) update.

This on the ebay forum suggests ebay may need to tighen up the way they serve pages.
https://community.ebay.com/t5/Selling/Now-What-Description-area-is-a-big-red-box-marked-Unsafe-Content/td-p/26063990

I suspect it's trying to detect 'mixed content' in iframes: you can google that but it's essentially an exploit used to inject malware into websites. But I'm guessing.