Is it permissible to take the price difference when buying products for a friend from China, knowing that I bear full responsibility for the quality and delivery of the product? And is a physical handover (qabḍ) a condition for buying silver necklaces online?

The price difference is the right of the principal and it is not permissible for the agent to take it without the principal's knowledge and permission. The agent's responsibility for the entrusted matters contradicts the nature of agency, except in cases of transgression or negligence. It is not permissible to take a fee for guaranteeing. If custom dictates a fee for agency, or if the agent is someone who typically performs such work for a fee, then the agent is entitled to a customary fee from the principal, but it is not permissible to take it without the principal's knowledge and consent. The sale of silver necklaces for silver, gold, or currency requires immediate possession according to the majority of scholars, while Ibn Taymiyyah and Ibn al-Qayyim hold that they are a type of commodity, and thus their sale is permissible whether immediate or deferred.