You can get items used for completing the constructions from your friends if you both have one and the same quest for constructing or upgrading the definite building. Usually there are 4 items to complete the building. When receiving such an item as a gift, your friend can send you one of the required items using Send in return option.
If the building has already been finished and the quest completed, the friend will have no ability to send these items while the item that you've sent will be replaced with passengers.
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