When there is a single company operating or in other words when a single company is buying stock from third parties and selling them to its customers transfer pricing is not applicable. Transfer pricing exists so that groups can set rules for the price that will be set when goods are sold from one member of the group to the other.
If transfer pricing did not exist, group members would be able to charge anything they wanted to help each member show a better financial performance. If a group member was not doing well, transfer pricing could be used to reduce the costs or increase the revenues. Of course, this is not correct and is far from showing a true and fair view. They are also tax implications as there are group companies that do not necessarily operate in the same country (different countries have different tax rules). For the above reasons, transfer pricing is quite important.
Remember for tax purpose and in many countries, when there is a intra group trade (one group member to the other), the sale is deemed to have taken place at market value (as if the customer was a third party). As said above though, each country is free to impose any tax rules they want.