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

To install the addon, follow the instructions from Installing Addons from the Marketplace.

Once installed, the addon automatically disables checkout from unauthorised customers and engages them to register at 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.