Use a laptop / PC not the app or mobile browser, and try Chrome, that seems to work well with eBay, then proceed this way:
- Go to Account settings in My eBay.
- Select Personal information.
- Select Edit next to Account type. You may be asked to sign in again.
- Select Confirm to upgrade to a Business account.
- Specify whether you operate your business as a legal entity or as a sole proprietor and select Continue.
- On the following page, enter your business information such as name, contact information, and returns policy. Then select Save.
Probably best to use a PC / laptop for this, and not the app or mobile browser. Chrome seems to work best on eBay.
You won't lose your feedback from your Private account, it will be transferred.
@mightymouse01420