*Important* Need gamers advice

Okay so let me begin with the back story in my town in England there is 3 game stores on one street GAME Gamestation and a recently opened GAMESLAND (stupid name i know) game and game station are pro's but gamesland are just rookies in this game so anyway it was Sunday and all the big time stores for games were closed and i had to trade a game in so i went gamesland since it was the only one opened and i decided to get Assassins CreedII  cause it costed £12.99 but i got £12.00 anyway so yesterday i go to gamestation and out of curiosity i ask how much is the game worth and they tell me £22.00 and i was like wtf so i know donno if i should trade it in now and get tekken 6 or another game or keep it cause dude its awesome :P help me gamers!!!