Blacksmith of Mammon is part of the Seven Signs event.
The Blacksmith of Mammon can be found in the various Catacombs (not Necropolis) at random times throughout the Seal-Effective period.
He travels every 30 minutes so you may have to chase him down before you catch him.
Unequip your items first before speaking with the Blacksmith.

You can perform the following tasks with him:

Exchanging Weapons
Upgrade Weapons
Unseal Items
Create Dual swords
Add Special Abilities

Merchant of Mammon is also part of the Seven Signs event.
The Merchant of Mammon can be found in the Necropolis at random times throughout the Seal-Effective period.
You will need ancient adena and/or blank scrolls to purchase his items.