So this is an issue which most likely has something to do with:
Basically eBay support helped me solve the problem of changing account type - or rather linking my eBay SandBox account to PayPal account, which now resolved the missing Account Type option on Publish Listing rest call.
However - now when I try to sign in to Sandbox using my existing account, I got no error but I'm not logged in either (there's still "Sign In" option in Sandbox header, at the top left).
Could this be looked at please? Support article for "I cannot log in to the eBay Sandbox" doesn't solve the issue for me: https://developer.ebay.com/contact-us
Sandbox account name: testuser_lukaszformela