When I try and post a listing on eBay it tells me I can't use
— but all the professional listings I see have iframes...
What's the catch? How can I add iframes to my eBay listings as well?
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 will be loaded fully embedded as part of the page. On others, it will be loaded in an iframe. Both will be using the same information.
An example listing viewed via Opera shows this in the source code:
However, the same while using Firefox renders this:
Same listing, different rendering.
you could try using simplebooklet.com. You can add your content via iframe into one of their booklet pages, then embed their booklet right into your ebay listing. It's a bit of a workaround, but it does let you do some pretty cool things in getting more engaging content on your listing.