Perfect Trade is a Coromon Quest you can pick up at Frostpeak near the town of Alavi.
You can find Abdallah in some snowy grass at the bottom of Frostpeak. He is after Perfect Coromon and wants you to trade him a Potent Scent x2 to help him find them.
You can find Potent Scent as loot around Velua. You cannot buy Potent Scent, however, you can also craft it in Soggy Swamp for trading in Swamp Shrooms.
It might be easier however, to just find some Potent Scent in Treasure Chests or ground loot. If you follow our Coromon Walkthrough, you can search for Potent Scent you can pick up naturally.
In return for the 2 Potent Scents, he will let you choose one of three Perfect Coromon.
All are Ghost type Coromon. They start at Level 25.