This system varies depending on the game.

Age of Wulin
You can have only one character per account, as of that, a trade between characters of other accounts will be as if trading with characters of other players (Direct Trade, Mail System).

Flyff
Each character has a vault to store items. This vault is accessible only by one character and the items can therefore not be shared between different characters. The in-game shop offers an item called: “scroll of common bank” which will allow you to trade items between the characters on the same server and account for a limited time (15/30 days).

Another solution is the mail system which allows for the sending of unbound items between characters. You can also launch the game client twice and thus use direct trade between the accounts that you use.

Rappelz
All items that are not character bound can be shared between the other characters on the same account and same server by placing them in the Warehouse. However you cannot share those items with characters on a different server.

Terra Militaris
Item trade can only be done using the Auction House or the paid-item transfer function. The mail system allows only the system to send attachments.