You can trade in your collected Seals at any Seal Trader in exchange for special items. Seal Traders can be found at the Protector's Enclave market, or at certain camps outside the Enclave.
Seal of the Drake used to be earned in elite dungeons or by trading 40 at the Caer-Konig Bounty Master for 1, yet no longer available.
These used to be exchanged in Protector's Enclave to Phileas Gram the Unicorn and Drake Seal Trader for Tier 2 Epic dungeon equipment. They were mostly removed from the game with the release of Elemental Evil Exchanging for Rough Astral Diamonds near the end of it's life until module 9 when the [Seal of the Adventurer] was added.