2. The purchase and sale contract must be stamped with the official seal to take effect. You are a self-employed, not a legal institution or organization, and have no right to sign a sales contract. Even if you do, it has no legal effect.
I suggest you be an import and export agent, so you only need to be responsible for customer communication. But please remember: you must sign an agreement with them that can protect your interests, otherwise people will directly go over you to contact customers, and you will draw water with nothing.
4. If you simply do foreign trade business in a certain industry, you can consider affiliated with a factory; But if it involves a wide range, it is still linked to the import and export agent. The proportion of profit distribution is only because of the industry, so it is not easy to answer.