Do all the tasks of the Sky Research Institute, then upgrade the Sala magic stone he gave you to level 5. After you go back, he will give you a rose card and open 4 A-level equipment with different attributes. You take the rose card to find the Black Merchant. There are B+ equipment in the Black Merchant, but there are not on the map. Moreover, the Black Merchant has Book of the World 7. It is best to complete this task of the Sky Research Institute first. The materials are very fast, and it takes some time to upgrade the magic stone. Otherwise, when you upgrade the magic stone to level 5, the equipment of the Black Merchant will not be of great effect.