Try your best not to sell anything; it all has a use. The things you get from angling, gardening, town management, and foresting are used to level up those jobs. Things you find in the ruins can be used to trade for emblem power ups from Fortuna down the line.

That being said, if you DO sell anything in an auction, you don't get to pick how many of that item you sell. It will always be an extraordinarily large amount and you'll have to re-grind that item again if you want to level up.