I've considered signing up for
http://www.vintagecardprices.com/. Is the amount of info available on the site worth the subscription fees? Is it mostly baseball stuff?
Seems like there's a lot of good info at collecthemall.com, thanks for the link. It's pretty similar to what I was talking about. I'd have to take some time to read it and digest it all.
I think a project like this would serve more than an academic purpose. If someone did this, and no practical information were gained from it that changed the way you bought, sold or priced cards, then that in itself is a piece of practical information.
But chances are you're going to find something practical in there. For example, if you track the prices of Maurice Richard RCs for three years, you may find that in year one there was a sharp decline in the sale price of PSA-5's, in year two they remained the same, and in year three there was another decline. You might be able to match that information up against the population report for that card, and maybe you'll find that each of the two years in which the card declined saw new Rocket Richard RCs graded higher than 5 added to the population. You could draw an inductive argument based on that stating that it's likely that the added population of high grades for that card have devalued Richard RC PSA-5's.
The information itself would be useless without subsequent analysis. It would basically just be for the premises for arguments pertaining to why certain cards have certain selling trends. Maybe you have no idea why the Richard RC's dropped so badly in price those two years, which opens up to tons of theories regarding the fluctation in price. The analyses gained from this info could easily effect the pricing of such cards.
The point I'm making is without the information, we can't estimate what that info holds. With that information, even if it's useless, then at least we would now know that. But without the sales stats, there's no way we can know because we don't have the info.