If you want to be sure that any of your buyers has a registered profile in your store use the Register on checkout module.

To install the module follow the guidelines from Installing addons from the Marketplace.

When installed, the module automatically disables checkout from unauthorised customers and engages them to register on checkout.

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There a customer will have either to sign in with an exisitng account, if he has it, or register a new account if this is his first order in your store.