A self-service chemists shop had a checkout at the exit. Dangerous poisons and drugs, which by statute could not be sold except under the direct supervision of a qualified pharmacist were kept separate in a special area. Customers were able to select the articles they wanted to purchase and pay the cashier for them at the exit. A registered pharmacist supervised each transaction at the exit, and the pharmacist was entitled to refuse goods to any customer. Discuss legal conditions of the sales contract