Sealing makes items that are normally untradeable, tradeable.
After sealing equipment, you may trade it once with another player, or place it in your Account-Shared Storage.
If you place it in your account-shared storage, the seal will never break, even after taking it out of storage, unless you break the seal yourself by equipping the item and confirming that you wish to break the seal.
Please be aware that not all untradeable items are sealable.
In order to seal, you must have a Quality Assurance Seal (품질 보증 씰).
Quality Assurance Seals may be purchased in UNION Store (3000 NX for 10), or from other players in the Black Market.
It's also possible to acquire seals via dungeon drop or through crafting.
The amount of seals required to certify varies, depending on the item's rarity/rank and grade (if any).