If a person buys a product and offers it for sale, and says, “It cost me such and such” but he is lying by saying that he paid more for it than he really did, then he is doing something that is haraam and is committing a sin. He risks losing the barakah (blessing) of his sale. If he swears by Allaah to that effect, then his sin is even greater and the punishment will be more severe. He is included in the warning mentioned in the hadeeth narrated by Muslim in his Saheeh from Abu Dharr (may Allaah be pleased with him), in which it says that the Prophet (peace and blessings of Allaah be upon him) said: “There are three whom Allaah will not look at on the Day of Resurrection, nor will He purify them, and theirs will be a painful torment.” We said, “Who are they, O Messenger of Allaah? For they have indeed failed and are losers.” He said, “The one who reminds others of his favours, the one who lets his lower garment hang below his ankles, and the one who sells his product by swearing false oaths.” (al-Tirmidhi said, this is a saheeh hasan hadeeth). According to another report, “by evil oaths.” And al-Bukhaari, Muslim and others narrated that Abu Hurayrah (may Allaah be pleased with him) said: “I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: ‘Swearing an oath may sell a product but it destroys the barakah.” 

Al-Bukhaari narrated in his Saheeh (vol 4, p. 316) from ‘Abd-Allaah ibn Abi Awfa (may Allaah be pleased with him) that a man offered him an item for sale in the market and he swore falsely by Allaah that someone else had already made him an offer in order to get a Muslim to buy it.  Then the aayah was revealed (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]

Al-Bukhaari and Muslim narrated in their Saheehs that Abu Hurayrah (may Allaah be pleased with him) said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “There are three whom Allaah will not look at and will not purify, and theirs will be a painful torment: a man who has extra water but he withholds it from the wayfarer; a man who gives allegiance (bay’ah) to another man (according to one report, to a leader) but he only did so for the purpose of worldly gains, so if he gets what he wants he is loyal to him, otherwise he betrays him; and a man who bargains with another man concerning some product after the time of ‘Asr and he swears (falsely) to him that he has already been offered such and such, and he sells it.”

(al-Bukhaari, 124, 160, 376; Muslim, 1/103, no. 108).