On your first visit to Treno you might take note of the Auction house but you probably can't afford anything there. However, on your later visits it pays to check out their selection. Bidding is simple. You simply walk up to the auctioneer and when the question mark appears above your head you can press X and set your bid. You don't want to buy accessories from the auction because they're much cheaper elsewhere. However, there are a few key items that are of interest to you. To add to your miniatures collection you can get the Mini-Cid. You'll also definitely want to buy Dark Matter, an accessory that allows Dagger to summon Odin. The other items of interest are Une's Mirror, Doga's Artifact, Griffin's Heart, and Rat-tail.

These are items you may want for your collection ultimately but they also have high resale value. What's better is they will reappear at the auction so you can buy, sell, and buy again to keep. The perspective buyers in Treno are the Adventurer who walks through the cafe and then up the stairs,the nobleman in the cafe, and the scholar in the synthesis shop. You'll want to wait until the bidding war slows down and then make a bid just higher than the last bid but if the bidding goes above their resale value go out and come back in. Once you have the item you'll want to track down the buyer. They'll give a first offer and if you decline they'll make a higher offer EXCEPT for the scholar. Here are the items and their resale values:

After you've bought and sold the above items a new item will appear at auction - the Magical Fingertip - which is used to acquire Excalibur. There is also another use for Une's Mirror and Doga's Artifact. During Disc 4 with them in your inventory go to the Black Mage Village and examine the gramaphone in the inn. The background music will change to a song from Final Fantasy III which was never released in the US.