To buy a legendary sword in Blox Fruits, follow these steps:
First, reach level 700 and complete the Military Detective’s quest. This will grant you access to the Second Sea.
Next, find the Manager at the Cafe in the Second Sea. Talk to him to get information about the Legendary Sword Dealer’s spawn time.
Wait for the Manager to inform you when the dealer will appear. The dealer spawns at regular three-hour intervals.
Once the dealer is about to spawn, search for him in one of seven locations in the Second Sea. These include spots like the Colosseum, the Green Zone, and the Remote Island.
When you find the dealer, you can purchase a legendary sword for $2M. Note that the dealer does not sell all swords at once, so you might need to wait for a restock.
To get the True Triple Katana, buy and upgrade Saddi, Shisui, and Wando. After reaching 300 Mastery on each, visit the highest Stem in the Green Zone and speak with the Mysterious Man. You can then buy the True Triple Katana for $2M.