TRADE
28.1 Players trade to build up sets of the same commodities, as sets are more valuable than individual commodity cards. Trade is open to all players. Offers may be suspended, altered or withdrawn in open negotiation between players, but once trade cards have changed hands, a deal is complete and cannot be revoked.
28.2 Trade is carried on by a system of barter involving only trade cards. Trade deals may not include treasury or civilization cards. Any one trade deal may involve only two players.
28.3 Each trade must involve at least three trade cards on each side. A player with fewer than three trade cards may not trade. When negotiating a trade, each player must honestly inform the other of the number of trade cards he wishes to trade and at least two of the trade cards involved in the trade. This information must be correct - the remaining card or cards need not be specified and may consist of any commodity or tradable calamity card(s), regardless of what was said to the other player. A player may not show his trade cards to another player during negotiations, nor may a player inform other players of the details of a trade after it is completed.
EXAMPLE: A player, wishing to acquire grain, announces 'I want grain -I'll trade salt for grain.' He agrees with a prospective trading partner that he will trade three cards, including two salt, for a grain, an iron and two unknown cards. He guarantees that the other player will receive two salt cards. An assurance that the third card is also a salt does not guarantee that the third card traded will not be a different commodity, or possibly a tradable calamity card. Similarly, our player can only be sure that he will receive one grain, one iron and two other cards. He has no way of knowing what the other cards will be until he receives them, although meaningless assurances can be given by his trading partner.
28.4 Limiting the trading phase
28.41 Trading is permitted to continue until all players have completed all the deals they wish to make. It is strongly recommended, though, that a time limit of not more than five minutes be imposed upon the trading phase.
28.5 Commodity Card Sets
28.51 When a player holds more than one commodity card of the same type, the value of the combination is increased according to the following formula: square the number of cards held, then multiply the result by the value of the commodity. For example, three grain cards (each worth four) have a value of 3 x 3 x 4 = 36, rather than 4 + 4 + 4 = 12.
28.52 The values of the cards when collected in sets is printed on each commodity card. Different commodities, even of the same value, may not be combined in sets.
28.53 Mining increases the value of Iron, Bronze, Silver, Gems or Cold by one card. This effect only applies once each turn. For example, if a player holding Mining uses three Bronze cards to purchase a civilization card, he could use Mining to increase the value of the set from 54 to 96 (the value of four Bronze cards), but could not also use Mining to increase the value of a set of Gems cards in the same turn. Mining may not be used to increase the value of a set of commodity cards beyond the limit printed on the commodity card itself.