I just found this out like 5 minutes ago, so it might not be a perfect guide and it might not work for everyone, but bare with me. So here's how I did it:
- Already be in a Roblox account.
- Go to the avatar editor.
- Log out.
- Click "sign up" on the top right.
- Create an account.
- Now you have an account with no inventory!
- If you're too lazy to do the first 4 steps, then you can just go here: https://www.roblox.com/account/signupredir?returnUrl=https://www.roblox.com/NewLogin?ReturnUrl=%2fmy%2favatar