Player-to-player trades can be made between any two cooperating player ponies.
To initiate a player-to-player trade, one of the involved ponies must right-click the other to bring up a menu. "Trade" must then be selected, and the other pony must agree to the resulting alert.
Once both ponies agree to trade, the trade menu appears. Each pony selects the items or bits they want to offer. Pop-up menus allow selecting a portion of a stack. If both ponies agree to the trade, the items and bits are exchanged (removed from the provider's inventory and deposited in the recipient's) and the trade completed.
Player-to-player trades do not involve set values and do not even require the use of bits. Items can be exchanged for items. And if both ponies agree to an exchange, there does not have to be any reciprocity -- items and/or bits can be gifted for little or nothing in return.