Is it permissible to swear an oath in buying and selling if the person is truthful?

Swearing in buying and selling is absolutely reprehensible (makrooh), whether it is false or truthful.

If it is false, then it is reprehensible to the point of being forbidden (makrooh tahreem), and its sin is great. It is considered an oath that immerses one in sin (yameen ghamoos) which eradicates the blessing of the sale, as stated in the Hadith: "Swearing promotes the merchandise but erases the blessing." The one who does so is among those whom "Allah will not speak to on the Day of Resurrection, nor look at them, nor purify them, and for them will be a painful torment."

However, if it is truthful, then swearing is reprehensible (makrooh) to the extent of being discouraged (karahah tanzih); this is due to the promotion of merchandise through excessive swearing. Allah has forbidden making oaths a shield, and it was narrated from the Prophet, peace and blessings be upon him: "Beware of excessive swearing in selling, for it promotes then erases (blessing)."