This is usually something we phrase to our clients in the form of a question.
"Will a sales order ever have more than one contact?" (many-to-many)
"Does every sales order have only one contact?"
"Will a contact ever have more than one order?". (one-to-many)
Once that's answered, if there is a need for a one-to-many relationship (Contacts have MANY invoices), then we will establish that relationship between the two tables, and add the portal necessary to fulfill the "many".
Why not have an Invoices or Sales area and just have a field that salesman that just populates with his name. sure you can have a portal of sales but this should just take you to a seprate invoice screen to create the sales of the products.