There is no catch as there are no iframes allowed for use in any eBay listing.
What you are seeing in some listings is how the server has chosen to render the listing according to the browser.
In some it ...
To do this, you need to combine the src strings from the embed codes. So if you want to display these two calendars:
<iframe src="https://calendar.google.com/calendar/embed?src=555555555555555555555%40group.calendar.google.com" style="border: 0" width="800" height="600" frameborder="0" scrolling="no"></iframe>
This is accomplished with the CSS rule
Note that Opera uses "-webkit-user-select" because this CSS is still experimental.
Extensions like RightToClick on Firefox override this. A userscript that overrides this CSS rule would also work.
I believe that this could be regarded (by Facebook) as a feature, rather than a bug.
I can't find it now, but am certain that I've seen a reference to it in Facebook's documentation, with a comment that the "fix" it to switch back to using FB as yourself instead of as a page.