20 million is a hefty piece of cheddar. But, in my HUMBLE opinion, this stuff is worth a lot more than 20 million dollars.
Anyway, I don't think they should sell it to the library of congress. I really do not trust those people. As we all know, records have a way of disappearing when the government is involved. I know that this is not material which is important to national security or anything like that, but I just feel like the Kind family should hold on to material (and maybe license its use).
With the licensing, I would hope that the King family would strongly regulate how the image of this man is used. For example, there is a McDonald's commercial that comes on every year with MLK's picture in it during Black History month. Personally, I don't think his likeness should be used in a McDonalds commercial (I got issues with all of the fast food commercials running during "our" TV time anyway)

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What's wrong with them licensing his image for use in a stay in school or get an education commercial. (I do not ever remember seeing one of these, but there may have been some done.)
This is just my opinion though.
Six_Three_Sigma