Where does it come from and how hard is it to get?
The Flag of Ownership comes from the World of Warcraft TCG (trading card game)
TCG Expansion: Drums of War (Press Release)
Rarity: 1/181 booster packs (speculative)
Distribution: Drums of War booster boxes contain 24 boosters and cost approx $65 USD (inc. shipping), therefore the average cost of obtaining this particular loot is $490.00
How much would I expect to pay for one off eBay or similar?
According to http://www.warcraftlootcards.com/owned/the_flag_of_ownership.asp (most information sourced from there), the expansion was released on the 28th of October 2008. Upperdeck the makers of the TCG have specified that all three loots in this expansion (Party G.R.E.N.A.D.E., The Flag of Ownership, Big Battle Bear) are as common as those in the previous set, so apart from the initial rush during release the price for this item should average around $40USD. This item item has special PvP appeal which raises the desirability from previous tier 2 loot.
The flag can be used on any slain foe (enemy players).
When used triggers a special sound effect and emote "<Your character name> has placed the flag of ownership in <Enemy> corpse".
Has a 60 second cooldown
Useable at any level and therefore available for twinks.
How do the Loot cards work?
Loot cards are special rare versions of WoW TCG cards found in booster packs, starter decks, and raid decks that have a revealable unique code. Once this protective layer concealing the code is scratched away, a 25 digit promotion code is revealed that can be entered into the Blizzard official promotions page. These codes work for both US and EU servers.
I have the 25 digit loot code, now what?
All accounts from every region in the world use http://www.worldofwarcraft.com/misc/promotion.html to redeem any and all special loot. This webpage allows you to enter the 25 digit promotion code and also select the region and realm for the character that will receive the loot item.
Upon successfully submitting the above information, you will be presented with an in-game code. Write this code down or print it out; you will need it to retrieve your item. Log into the game with the character that is to receive the item and travel to Booty Bay in Stranglethorn Vale. There, you will need to find a goblin by the name of Landro Longshot. He is located on the lower parts of Booty Bay on the docks, and should be easily recognised due to the fact that he has a distinctive purple shirt and two body guards wearing Tabards of Flame next to him.
When you find Landro, he will first ask if you are here for a promotion, and then ask you which TCG expansion your item comes from (or Points Redemption); check your loot card if you are not sure which item that is. Ensure you have got bag space to hold the item!