While adventuring around other Islands you will encounter Trader Rahul. He is looking to purchase rare materials from you and offers good prices for them. You can find more of the rare material by tapping on the question mark tip button.

Watch out - he doesn't stick around forever! He has a timer, so keep an eye on it. Once the timer runs out, he will disappear. He will also disappear if you have completed all of the trades he has to offer and you will get a bonus for completing all the available trades. You can tap and hold onto each item displayed in his backpack to see the names of the items.