If you own both a production company and a retail company, you cannot buy with your retail company inventory from the production company.
I think this setting is wrong as in real life an action as this one is possible. A person might want to do this in order to make a greater profit as you may decrease the costs of buying inventory for the retail society. You could do this buy lowering the price of the product you produce. This way the retail buys products at a lower cost and sells them at the market price increasing the profit.
It is true that you must cover the production cost of the product but as long as you keep the price of the product equals to the production cost you will make 0 profit with the production company but increase the profit of the retail company.
In real life this kind of market operation exists and helps companies make money. By not letting the users of GoVenture World do this the site blocks our mindset and our profit possibilities as well as differentiates the game from reality, thing which should not happen as the game must be the most similiar possible to reality.