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Dave SGuides are indeed that...guides..and as Eric said, intended to put you in the ballpark. Unfortunately, some ballparks are bigger than others..as are card discrepancies as to prices and values. If you live by price guides, you'll lose a heckuva lot of cards for a few bucks and likewise overpay on some. Bottom line is what is the card worth to you and your collection. And keep in mind that no 2 vintage cards are alike...which furthers the skewing of price guides.
As for Auction/Market services...and please someone correct me if I'm wrong...the problem I see is that most do NOT account for the thousands of cards that go unsold because reserves weren't met or auctions were ended "unsold" early when in reality the card could've been sold...