When doing business in China, India or Vietnam you could use all the help you can get. It is a logical step to get a local sourcing agent that can help you with finding factories and ease the communication with suppliers. It is therefore not strange to think that an agent can also help with quality control. However, these local sourcing agents are not suitable for inspecting your products. They are not educated quality engineers, so they cannot perform a professional inspection. You will not receive an inspection report. How can you be certain that the inspection was actually performed? You cannot ensure that the sourcing agent was in your factory. Maybe he was busy or at the other side of the country. This accounts for a lot of risk, for which the agent cannot be held responsible.
Most agents will still claim to be able to perform professional quality control. It is possible that your agent is actually a real inspector. But it is normal that the sourcing agent receives a commission from the factory of which the importer is unaware. An agent gets a double pay: from the importer and from the factory. Therefore a sourcing agent will never reject an order, because the conflict of interest.
In short it is better to perform the quality control yourself or have it done by an independent Quality Control company.