To answer all questions, there is no trading allowed between two accounts. You can have 3 characters and you're allowed to trade items with them, but there is no way to trade items or anything through another account.

There are multiple "currencies" in the game technically.

You have legendary marks-which are received from legendary(purple) items with the Taken King logo on it, and they are obtained through other activities. The main currency is glimmer, and that is what you use to buy things. You can use that with also use of another type of currency in the game. with this items, you are not allowed to trade with anyone with any of the items you bought with any currency.

Note: Certain items will only require certain currencies so understand that.

Hopefully this helped you out a little bit!