Exchange is the voluntary change in ownership over an entity or entities between two owning entities. The exchange can be symmetrical where there must be parity between the values of exchange, or asymmetrical requiring no such equity. In symmetrical exchanges, the entities exchanged may be similar in function, or completely different. It is still symmetrical as long as the values of the entities are equal.
Exchange with player-controlled entities require both players to agree to the exchange. These exchanges have no required symmetry; instead, the players determine "fairness" of an exchange based on their strategy. Exchanges with non-player entities are often forced to be symmetrical by the the system. For example, in many RPGs, players can shop for items using currency. The amount of currency exchanged for an item is set; the non-player entity will not take more or less currency for an item. Some games still do not require symmetrical exchanges between a player and a non-player entity. In these games, players can barter with non-player traders to get the best deal when buying or selling their goods. Symmetry again becomes relative as it is in player-to-player exchanges.
In Neverwinter Nights, player-controlled entities can exchange items with each other. The exchange process is initiated by one player. Both players select items to offer, then press the "Offer" button. If they change their items, they must press "Offer" again. Once both players have pressed "Offer", each player must press the "Accept" button to finalize the exchange. The exchange can be asymmetrical or symmetrical.
In Pokemon, players can exchange pokemon with each other. The exchange is always a 1 for 1 trade. Each player selects which pokemon he wants to give away. Each player is then alerted of the pokemon he/she will be receiving in exchange. If both players accept, then the exchange takes place. Additionally, the Pokemon received retains the name of its original captor. The game has explicitly designed trading as an important aspect since there are some Pokemon who only become available as the result of a trade taking place. Different versions of the game have only partial subsets of the Pokemon available in the game. Additionally, some pokemon are transformed (they evolve) by the trading process.
In Final Fantasy III (US), the player-controlled entities can exchange gold for items with merchants. Different merchants require different amounts of gold for the same items, but each merchant will only accept their set amount of gold.
In Trade Wars, player-controlled entities can exchange currency for goods with non-player entities and vice versa. The non-player entities have a starting price, but the player-controlled entities can try to haggle to get the best deal. The non-player controlled entity may or may not accept a player-controlled entity's offer
In Yahooligans! Go Fish!, if player A requests a card of rank R from another player B, and player B has one or more cards of rank R, then player B transfers all cards of rank R to player A. The exchange is voluntary in the sense that player B wants to continue playing the game; if player B did not transfer some of those cards to player A, player B would cease to play the game. The exchange is involuntary in the sense that player B does not want to increase player A's chances of creating additional books (4 cards of the same rank). The player with the most books wins. If player B had more books than player A and knew that player A had a low chance of creating a book from his cards of rank B, player B might volunteer to transfer those cards, since the game ends when a player has no more cards or the deck has no more cards.