A clansman wandered into the village by mistake, encountered a Pseudo-Dragon, and obtained a terrain map [get it?].
The trip to Dragon-Trapping Valley was meant to change the eldest maternal uncle's fate, thereby revealing what had happened twenty years earlier and laying the groundwork for the Longlife Tribe's traitor. At the same time, Toothless gained intelligence, and the dragons' hidden grounds were obtained. Seventh Sister also gained an understanding of reincarnation and cultivated the Phantom Doppelganger Art.
The auction was there entirely to unravel the mystery of Seventh Sister's origins and the blood oath, as well as the Dragon Clan Lands [get it?].
If you still don't get it, I can only blame my limited ability. I can't write something with that kind of sweeping grandeur, so I can only write this carefully according to the outline.