Roblox have extensive info on their website about creating/reselling their copyrighted items, so your listings must be compliant with them.
You're trading as a business but doing it on a private account (which is for sellers listing their own personal items such as bits from their wardrobe/attic etc. eBay will be passing info on your sales on to HMRC anyway, so it's vital to keep accounts.
To correctly register as a business seller simply go to your Personal Information in your account and to the right of Account Type, which will be showing as 'Individual' you'll see an Edit option.
https://accountsettings.ebay.co.uk/profile
This doesn't affect your feedback profile or any current listings, it merely upgrades your account so that you're compliant with current consumer and eBay policy.
If you're also selling your own items, just open another eBay account (you can have more than one).