Abu Qatādah (may Allah be pleased with him) reported that he heard the Messenger of Allah (may Allah's peace and blessings be upon him) say: "Beware of excessive swearing (by Allah) in sale transactions, for it sells (the goods) then erases (the blessing)."
Beware of swearing too much when buying and selling, even if you are truthful, because excessive swearing most likely leads to lying. For example, a person should not say: "By Allah, I bought it for one-hundred," even if he is saying the truth. If he said: "By Allah, I bought it for one hundred," while he bought it for eighty, then this is even worse, because he is lying and, at the same time, swearing in a sale, which was forbidden by the Prophet (may Allah's peace and blessings be upon him). He informed us that though swearing oaths in buying and selling is a means for selling goods, Allah, the Almighty, then removes the blessing from the sale transaction because it is based on disobedience to the Messenger of Allah, and disobedience to him amounts to disobedience to Allah, the Almighty.