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davidcyclebackYou might wish to try the website www.gameuseduniverse.com It has a chatboard similar to here, except is used by game used collectors. They're more likely to know about game used hats.
As with autographs and baseball cards, there is a lot of trickery and fakery in game used, so it always is best to be careful and skeptical.
From my point of view, I would want to know who is offering you the hat. There are a number of reputable and knowledgeable game used dealers and collectors. If one of these was the seller, that would be significant info.
I agree with you in that I also would want, if not require, some documentation or proof of said Tresh provenance and Mantle story.
Best of luck.