The auction house is where researchers trade specimens they have captured in the field. Players list a captured cryptid, set a starting price and an optional buyout price, and other researchers bid on it with in-game currency until the timer runs out. When an auction closes, ownership of the specimen moves to the highest bidder, and the record on the researcher's profile moves with it.
Listings only exist while the game is being played, so the floor can be quiet between expeditions. Every specimen is unique: the illustration and the field report are generated for that one individual, which is why prices vary so much between species and between captured and merely documented specimens.
Listings appear here as soon as a researcher puts a captured specimen up for sale. Try a different search, or head to the discoveries feed to see what has already been documented in the wild.