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MattVintageCardprices.com does provide facts, but they are somewhat inflated. Any sale that doesn't take place via auction isn't there. The difference is significant - take for example an ebay listing that sells for $1,000. After ebay and Paypal fees, say the seller only sees $925 (something like that) of that. At a card show, that deal gets done for $925 cash. I don't know how VCP handles the "juice" from auction houses but that could be an even more significant difference %15-%20. So, part of what gets rolled into the card's value on VCP is the cost of advertising and running the auction, which, of course, is not part of the value of the card at all.
You could make a case that the only thing relevant to the card's value is how much someone is willing to pay for it, and since the buyer pays the "juice", that should be included in the value of the card. Even so, the fact that the same transaction occurred for $925 at a card show and that is not factored into the VCP average is a flaw. If the VCP prices were only used for determining auction prices, then maybe it's no problem, but when VCP prices are used for transactions such as here on the BST, then it's inaccurate.
If I am mistaken on any of my facts, please correct me.