Liang the Dragon Merchant is a special merchant. He'll drop into your realm in his flying balloon and stay for a few hours. Once the time is up, or his needs are met, Liang will go ... wherever it is that dragon merchants do go ... and come back again in four hours.
Liang always asks for a few specific things. He gives you the same Coins and XP as Myra. If you fulfill all of his requests, he'll also reward you with a shiny pearl, either white, black, or gold. Pearls are used to entice dragon and unicorn heroes into your realm; each one costs a certain quantity of white, black, and gold pearls.
Dragons and unicorns make great heroes, and can bring good fortune hunting in Places of Legend, so it's worth devoting some effort to complete Liang's requests. But you won't always succeed.
Liang always wants three different items, arranged in two lots each, for a total of six separate sales (until you graduate to preparing nine: three lots of each item). You may conduct each sale as you are able, and that will clear room in your inventory. Usually the first item will be quick and inexpensive to produce, but you will need to provide a large quantity: up to a fifth of your storage vault capacity for each lot. The third item will be expensive or time consuming, but you will only need one per lot. The second item will fall somewhere in between those two extremes. This is a tendency, not a rule.
Sometime Liang will request items that take many hours to craft. If an item requires 12 hours to make, and you need three of them, Liang will be long gone before you finish them, even if you have two of that workshop. There are four things you can do to prepare for that. First, if you click his location while he is gone, he will tell you the three things he will want when he returns. Secondly, you might get some items through Sea Trading. Thirdly, if you are in an alliance, your alliance members might help you with difficult items or quantities (make sure you help them, too). Finally, you can build more than one of each Workshop or Resource, using crowns.
Note that once Liang arrives there is an option to "Change Liang's Order". If you choose this Liang will leave and come back in 4 hours with a different order. But you lose this option if you make any sales to him, so make sure you want to try to fill the whole order before selling anything to him.
If you don't manage to complete an order for Liang, don't worry. Liang will come back ... in four hours.