you cannot establish categories.
Only store sellers can do that.
However, you can certainly establish other accounts.
Here's how to open other accounts:
http://pages.ebay.com/help/policies/multiple-accounts.html
You can sell a variety of things on your existing account, without separate categories or accounts.
Many sellers do that.
However, as a new seller, you will have selling limits.
Read about them here:
http://pages.ebay.com/help/sell/sellinglimits.html
Not allowed
Registering new accounts or using other existing accounts to avoid buying and selling restrictions or limits or other policy consequences.