I stumbled upon this article this morning:

GeekDad Exclusive: Auction Preview of D&D Co-Creator's Personal Collection and Archives — Game's Secrets to Be Revealed - GeekDad®

which details how Dave Arneson's gaming stuff was found:

"Letters from Gary Gygax. Other letters, notes, documents ..."

"unpublished manuscripts that did not make it into the final draft of Dungeons & Dragons that date as far back as 1973."

"There are even older items from 1971 and 1972 "dealing with the Blackmoor campaign and the Castle itself," Stormberg said. These may reveal secrets about the game's origins."

"There are also runs of Gygax's Castle & Crusade Society and Domesday Book newsletters"

"Arneson's Corner of the Table newsletter which discusses his various campaigns, including his Blackmoor exploits"

"Some 10,000 items comprising what is being called the Dave L. Arneson Collection — ranging from Arenson's 1959 game of Risk to game designs he tinkered with up until his death — will finally be sold at a series of eBay auctions beginning Sunday, May 6"

Stormberg said that "About 30% of the items are what I call product: published games, game accessories, periodicals, and books." The remaining 70% of the collection is "non-product": all those letters and scribbled notes, maps, objects, and personal and family items."

There are "just so many treasures," Stormberg said.

[EDIT: I talked to Paul Stormberg about this and he told me over the phone that only Dave's items that aren't unique were sold, like old games and such.]