Before PSA and grading companies

[MattyC]

I'd venture that the advent of the internet is what's really responsible for being able to see the exact card that sold and the price.

Even without grading companies, if the cards were selling on eBay raw and there was a VCP-style database that logged in all those sales, one could likely find an exact card due to its centering, corners, and other unique flaws. Of course, the internet combined with the grading helps make things much easier.

That said, past sales data points aren't always that valuable, especially for rarer cards with a sparse sales history, cards that haven't had a pretty example sell in a while, or if you are trying to pry a specific card from a collector's collection.