Need some tu? Auctions are perfect! You bid (or choose the Buy Now option, if available) on a creatu. Then you release it in the forest in Atquati. You get tu whenever you release a creatu!
You might not get all of the tu you spent on the auction back from the forest...
"I am a ghost Of a girl That I want to be most.
I am the shell Of a girl That I used To know well." ♫♪♫♪
You get 50 tu per pet you release into the forest. So just make sure you're spending less than that :) If you're hatching and get a natural you don't want, hit the release button and you'll get 1000 tu. Better than naming it and putting it in the Forest.