From time to time, merchants will appear in the towns and offer interesting deals. Take a look at what they demand and offer you in return. It's worth it in most cases.
There are three different types of merchants in total:
Game Start: At the beginning there are 6 consecutive quests. You have an infinite amount of time for these quests. However, you must complete one quest to unlock the next. These quests are very rewarding.
Legendary Quests: Legendary personalities offer you particularly strong manifest cards as a reward. You have 48 hours to complete each quest. If you don't complete it, the quest disappears and you have lost the chance to get the reward. As the manifest cards are really extremely strong, you should definitely complete these quests.
Merchant Quests: A merchant appears in one of the towns every 8 hours. You have 24 hours to complete the quest. The rewards are either random manifest cards, goods or resources such as gold, reputation, research points, diamonds or pearls. Decide for yourself whether these offers are worthwhile for you.