As in Edit found the portal access, also Related Partner > Customer

User can add order, or purchase. But using our suppliers as option to buy. Instead it be necessary to place order internally (sort of) client places order with us. Not with our suppliers. (select supplier option, is not something I would like my customers to see)

 Hi M Grmn,

W.r.t your query, I have gone through it and tried to replicate it. Few recommendations which I would like to suggest you which might cater to your needs are as follows:-

1) Yes its possible to allow a customer to place an order at any given time by granting them access rights.

2) The configuration which you need to follow is that by Signing Up an external user. This can be done by Settings>Configuration>General Setting>Portal Access>Allow external users to sign up

3) After doing so we need to the login page and SignUp ourselves with inputs required.

4) Now once you log in with the user (the one which you had signed up), you will see that no menus are visible to the user(s).

5) For this we need to login with admin and go to Settings>Users>Users>Select the user created>Related Partner>Sales and Purchases>Customers. By selecting Customer in the last step we have made sure that the User created is a Customer

6) Also we need to configure the Access Rights under Settings so as to make the menus visible to the User as required.

Hope it helps

 Cheers!!