The process of registering an eBay sub-account involves several key steps. First of all, it should be clear that the application for a sub-account must be based on a legally registered corporate account. If your account is a corporate account, you can apply for a main account and a sub-account. But if your account is a personal account, you need to maintain the account first. When applying for a sub-account, you need to go through legal filing and eBay merchant account authentication, and you need to associate it with an already authenticated payment account.

The specific application steps are as follows:

1. **Legal registered corporate account**: First, you need to have a legally registered corporate account. If your account is already a corporate account, you can continue to apply for the next step.

2. **Sub-account application**: On the eBay platform, you can try to apply for a sub-account. This allows multiple operators to operate accounts at the same time. When applying for a sub-account, you need to provide all necessary supporting documents and be authenticated by the eBay platform.

3. **Associate with the main account**: The sub-account needs to be associated with the main account, and all corporate accounts can apply for two sub-accounts.

4. **eBay customer service assistance**: If you encounter any obstacles during the application process, you can contact eBay customer service and explain your situation and requirements in detail.

5. **Online application**: Application for a master account can be made through eBay's corporate entry channel, which usually requires submitting an online application.

Please note that the eBay platform allows sellers to apply for sub-accounts, but only for corporate accounts. If you are a corporate seller and want to apply for multiple sub-accounts, you need to explain the reasons and requirements to eBay customer service. Generally, one main account plus two sub-accounts are sufficient for business activities.

The above information comes from the official eBay website and the e-commerce website Zero One E-commerce. During the application process, if you encounter specific problems, it is recommended to contact eBay customer service directly to obtain the most accurate guidance and help.