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jayBoard members. You have got to know this. PSA, Mastro, Evans, SGC, GAI, they all need each other. Each Auction house that you have mentioned use their grading services. They all need & i'm sure they all work together on many things for REVENUE reasons (that's for a later date). Of course they are going to agree with each other since they are all feeding on one another.
PSA, GAI, MASTRO catalogues, Leland's have all made mistakes in grading, descriptions, items. Do the research you will see (like Mr. Verkman mentioned) Why is it that sometimes you see items withdrawn from an auction?
You have to take this out of the hands of these people from this hobby. They are all together and will agree together like a large conglomerate family. A private third party outside the sports memorabilia business will give you a better one on one definitive answer.