Swearing oaths in buying and selling is makrooh is general terms, whether one is lying or not. If a person is lying in his oath then it is makrooh in the sense of being haraam; the sin is greater and the punishment is more severe, because it is a false oath. Even if it is a means of selling the product, it erases the blessings of the sale and profit. This is indicated by the report narrated by Abu Hurayrah (may Allaah be pleased with him) who said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “An oath may sell the product but erase the blessing.” Narrated by al-Bukhaari and Muslim in their Saheehs, this version was narrated by al-Bukhaari. See Fath al-Baari (4/315). And it was narrated from Abu Dharr (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “There are three to whom Allaah will not speak on the Day of Resurrection, nor will He look at them or praise them, and theirs will be a severe torment.” The Messenger of Allaah (peace and blessings of Allaah be upon him) repeated it three times, and Abu Dharr said: “They are lost and doomed. Who are they, O Messenger of Allaah?” He said: “The one who lets his garment hang below his ankles, the one who reminds others of his favour, and the one who sells his product by means of false oaths.” Narrated by Muslim in his Saheeh (1/102); as similar report was narrated by Imam Ahmad in his Musnad.
But if the oath in buying and selling is sincere, then it is makrooh in the sense of being discouraged, because that means that he is selling his product and encouraging the buyer to buy it by means of swearing oaths repeatedly, and Allaah says (interpretation of the meaning):
“Verily, those who purchase a small gain at the cost of Allaah’s Covenant and their oaths, they shall have no portion in the Hereafter (Paradise). Neither will Allaah speak to them nor look at them on the Day of Resurrection nor will He purify them, and they shall have a painful torment”
[Aal ‘Imraan 3:77]
And because of the general meaning of the words of Allaah (interpretation of the meaning):
“And protect your oaths (i.e. do not swear much)”
And because Allaah says (interpretation of the meaning):
“And make not Allaah’s (Name) an excuse in your oaths”
And because of the general meaning of the hadeeth narrated by Abu Qataadah al-Ansaari al-Sulami, who said that he heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “Beware of swearing too much when selling, for it sells (the product) then it erases (the blessing).” Narrated by Muslim in his Saheeh, by Ahmad in al-Musnad and by al-Nasaa’i, Ibn Maajah and Abu Dawood. End quote.