The value is relative to the current demand. Simply put if more than 1 person is interested in the same card they will bid against each other to whatever they are willing to pay. We don't know the final value ahead of time, hence there are price guides which can give you ballpark values. There are many factors associated with value. Condition, age, print or real photo, used or unused, even the publisher, stamp affixed, postal cancellation etc. can affect the value. I find the best way is to place the lowest price possible you can afford to sell it for, on the card and let the current market demand dictate the price. At auction the name of the game is to get as many postential bidders interested in your card as possible and get the bids. If the card does not get a bid, just put it away and try again next year!