The essential concept is the same across all these systems and is about what access the user has and what they will be able to do. The important takeaway for single sign-on is to make it as seamless as possible to persist the information they have already input and they are already managing for this external provider. And the external provider is trusted and has single sign-on authentication and authorization.
So, the user then needs to be able to go to the eCarketplace system and go through the registration process and have a seamless experience when they are selecting single sign-on. Whenever you are working with an eMarketplace provider that provides single sign-on, you want to make sure you work with a company that has worked with Okta, Azure, Salesforce, OIDC, other systems, and other providers who utilize OIDC, such as Google, Microsoft, Active Directory, etc., and be able to integrate others like Twitter, LinkedIn; the list goes on.
Ultimately, being able to offer these different single sign-on providers so that the user has a convenient set of options. But they can manually input their information too. For most companies, it makes sense to allow them to manually create an account or leverage single sign-on.