The phrase is really used: «"Juniors Wailing" performed live in front of a sold out crowd at Wembley by the original Frantic Four, reunited on stage for the first time in 32 years», as description of a video posted by "OfficialStatusQuo".
I was surprised, too, and wondered if it was a mistake, that's why I searched the phrase on the Net and found this forum.
Although the term "sold out" properly applies to the event, in "sold-out crowd" it applies in fact to the attendants to the event. This kind of shift is called a "metonymy" - as a rhetorical figure or as an instance of language change.