Go to the player's "Private Trade" screen via their profile and drag the item to the screen, type in the amount you want for it "$0", that makes it show up as "Free" on the receiving player's screen. They will get a Private Message telling them of the offer, they can then go to their Private Trade screen and accept the offer. you will get a message when the item has been given, and the item will be deposited in the other player's inventory.