Heritage used to be the only one who would have multiple cards or sets sometimes in the exact same grade but Robert Edwards the last couple of years has surpassed them by a long shot.
As far as where a 50K card goes, we will each have our list of top choices but I don't think any of them have a list of secret bidders. I think one that is known for selling 50K cards on a regular basis is imperative. And making sure they won't have another example of your card in similar grade should be agreed upon in advance.
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