eldavojohn: I think we just have different opinions here. I'm not going to be able to provide the proof you want. My opinion is based on my experiences and patterns I see.
You seem to think that "bubble" requires some aspect of fraud, which may be the textbook definition (I'm not a student of economics), but in terms of informal usage I don't see how that is a necessary component.
Supply and demand is at work here as it is with any market. Even fraudulently built bubbles collapse not from exposure of the fraud but the decline in demand. So I don't see how pointing this is involved somehow doesn't make this a bubble. Perhaps people keeping the prices up are no longer buying, the popularity of the artists has decreased, or there are just too many sketch cards available. I don't have the exact answer.
As stated earlier, I'm using bubble in the sense of a rapid decline in value. If that is the incorrect terminology, then okay it's not a bubble. Maybe you're right--this is just a normal market correction and there's nothing to see here.
Collecting: artistic and traditional comic art sketches