If the buyer does not pay the price of the the goods that you asked him to purchase, do you bear the responsibility?

If you inform the store owner that you are an agent for another person and are taking the goods for them, then there is no blame upon you if the principal does not pay the price. However, if you do not mention that, then you are responsible for guaranteeing the price to the seller if your principal does not pay it. You can then seek reimbursement from your principal if you have paid it.