How can I send items in-game?

There are only two ways to exchange items with another player in Diablo III.

1. If you are physically in a game with another player, you can drop an item on the ground, and they can pick it up.
2. You can list something on the Auction House. You cannot purchase items with anything other than gold or real money. (No item for item trades)
3. (Personal Use Only) If you store something in your stash, your other characters can take items from it.

So for now, there is no real way to trade items (in a way protected from potential scams).

You can trade items between your own characters through the stash, which is shared, along with gold, across your account.

For trades with other players, you can use one of the auction houses (real money or gold), or trade while in an active game.