As ShiftyCake points out, vanity items is a very good way to make revenue in a F2P game.
Selling items that directly effects a players ability in the gameworld is risky to say the least.
A reason why many people are wary of F2P games are because of the unbalance these purchasable items causes.
Some of these F2P games are even refferred to as a P2W (or Pay to Win) games.
I must say that I dislike the way you are talking about your potential players.
how to make user eager to buy paid item continuously? how to make them fanatic?
If your aim is to make "users" fanatic in order for you to make money, I suggest not making a game at all.