Seraph Crystals were introduced in the Captain Scarlett and her Pirate's Booty DLC. They are a special form of currency, similar idea to the Eridium, and can be exchange for all sort of weapons and items of Seraph (light pink) rarity.
Seraph Store Location
You can exchange Seraph Crystal for Gear of Seraph Rarity.
How to get Seraph Crystals
Kill Master Gee and Hyperius the Invincible (Raid Bosses) for Seraph Crystals. Those are the "Seraph Guardians"
You can get up to 15 Seraph Crystals each time you kill them
Cost of Seraph Weapons
Each Seraph Weapon costs 120 Seraph Crystals
Seraph Relics cost 50 Seraph Crystals
Example of Seraph Market Inventory
Intense Devastator (Seraph Pistol)
Red Text Effect: Hail
Proactive Actualizer (Seraph SMG)
Red Text Effect: We need to talk about your DPS report
gaa dunk ga Ahab (Seraph Rocket Launcer)
Red Text Effect: for hate's sake
Gentle Retcher (Seraph Shotgun)
Red Text: Ewww
Deep Seraphim (Seraph Assault Rifle)
Red Text: Holy? Holy? Holey!