On the Sales tab of a Product you can find a place to add a Warning on Sales Orders:

Then at the top of each Quotation and Sales Order the Warning will be shown:

Hi,

Create a record rule on the product.product model.

Example domain to restrict products per customer:

    ['|', ('customer_ids', 'in', [user.customer_id.id]), ('customer_ids', '=', False)]

Add a Many2many field on products for allowed customers (customer_ids).

Create a record rule so only products where the current SO customer is in customer_ids can be selected.

Hope it helps.