Past sales are used in every market place. That's why price guides have developed over the years. Whether you're talking about cars, houses, or baseball cards, we have used price guides. The guides aren't perfect. They are only guides to help you arrive at a price. Most of the guides used to only come as monthly updates, but now you get nearly daily updates on the web based guides. But the accuracy of these guide depends on the amount of data fed into the guide's database. But if that's not fast enough for you, I guess you can look at the latest ebay sales.
If you aren't going to use some kind of past comparison, how do you ever have any idea what to pay for an item? If you find yourself saying, "This card just sold for x dollars last week on that online auction. I can't sell that cheap." Well, then you are basing your value on past sales.
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