After getting the airship, return to the auction house in Treno. In addition to various pieces of equipment, four Key Items can be purchased here: the Rat Tail, the Griffin Hat, and two Dolga and Une items. Buy all four of them (by the end of the game, you should have plenty of money) -- if you can't read the Japanese text, just look for items whose names you don't recognize.

   Cross town to the open-air cafe. You can sell all four of the items here by talking to the various people. The fat guy in red will buy the Uni item, while the woman in black will buy the Rat Tail and Griffin Hat. Decline their first offer and accept their second for extra gil. (If you decline the second offer, they'll refuse to deal with you; leave the screen and return.) Finally, the scholar inside the blacksmith's shop will buy the Dolga item, though he'll only make you a single offer for it.

RARE Rat Tail L@@K!!!!

   Once you've sold all four items, return to the auction house. If you want to complete your collection, you can buy back the four items here, though they don't seem to have any other purpose. A new item, the "something" of Magic (it's apparently related to Gogo), also sometimes appears; buy this one. It'll cost you a pretty penny (possibly over 50,000 gil), but you've just made plenty by selling off the other Key Items.

   Fly to Daguerreo and ascend to the second floor using the elevator on the right. Talk to the old man in the blue coat in the library and choose the first option to give him your newest Key Item. After a bit of speech, he'll hand over the Excalibur sword. While it's not as strong as the Ragnarok, it can teach Steiner the very useful Climhazzard attack.