Mytra - Heavens Tear Arc User

You cannot do that. That was an options from the MY-EN version of the game. In the International version you need to fight higher level creeps and find higher level resource points to receive higher level materials.

In all of the main cities you have an NPC called the 'Auctioneer'. Look him up and talk to him. One of his options is buying and selling throught the auction house.