Silsilah as-Sahihah 1136 — Why is a trader sinful?

Sayyiduna Abu Sa'id (may Allah be pleased with him) says: A Bedouin passed by me with a goat. I said: Will you sell it for three dirhams? He said: No, by Allah! Then he sold it anyway. When I informed the Messenger of Allah (peace and blessings of Allah be upon him) of this, he said: "He has sold his Hereafter for the sake of this world."