To unseal an Item, a player must go to a Bartender NPC and with the necessary items.
Players can find out what items are needed to Unseal an item by speaking to a Bartender NPC and selecting to unseal, putting the item from inventory into the middle box of the window that pops up, then selecting "Unseal".
Usually the requirements for unsealing are a certain amount of Gelt and a Seal Dissolution Pearl. If the player does not have all required items, he/she can usually obtain them by either fishing or buying the item off of another player.
Having all of the requirements usually unseals all seals on the item.