To buy antibiotics with the aim to overcome flu or cold is an extremely common and huge mistake. In fact, antibiotics help only with bacterial diseases, and each disease is treated by a separate type of antibiotics, therefore, only after an examination by a doctor, the patient can really know which medicine is worth buying.
In the same way, there is no sense to use antibiotics for prophylaxis, because in this case, it is also impossible to predict - the prevention of what illness exactly?
So it's not recommended to buy antibiotics without a doctor prescription. And even if you want to buy antibiotics online you should still follow your doctor's instructions. Don't waste your health and money!