Posted by the always-informative Doug Winter, and coins I likely will never own; unless, maybe, Publisher's Clearinghouse comes knocking at my door. This article cover dollars, quarter-eagles, and three-dollar gold coins, available here.
My favorite obscure US gold coins were the one and four pound coins minted in 1945 to pay the Saudis for oil during the war.
ARAMCO Gold info US Mint ‘Gold Disks’ Made for Oil Payments to Saudi Arabia from CoinLink [3/22/2010] Saudia Arabia Gold for Oil from silvergoldarabic.blogspot.com [11/1/2010]
Heritage Auctions had this 4 Sovereigns gold coin, lot #22749, listed April 23 2010. It sold for USD$2070. Besides the occasional auction, the other likely outcome was speculated in the quote given in post #5 of this thread.