What is the ruling on buying from Chinese merchants in light of China's stance on Muslims in Syria and its people, and is it permissible to set the price of products sold online without knowing local market prices?

The summary of the answer to your questions is in three points:

1. It is permissible to trade in lawful goods with disbelievers, even those at war, with the exception of weapons and similar instruments of war. 2. The ruling on commercial boycott in these situations is subject to the assessment of benefits and harms, and its ruling changes with the change of time and circumstances. The estimation of this is referred to those deeply rooted in knowledge. 3. It is permissible for a merchant to set the selling price as they wish, and there is nothing in Islamic law that obliges them to a specific profit margin that they cannot exceed.