Linguistically: It is the opposite of buying, and the two terms are used synonymously. [1] Al-Raghib said: “Buying and selling are intrinsically linked. The buyer hands over the value and takes the valued item, and the seller hands over the valued item and takes the value. This is if the buying and selling is of a physical good or commodity. If it is a trade of one good for another, however, then it is possible that both parties play the role of buyer and seller, and this is how the words became used synonymously.” Taj al-`Arus by al-Zabidi (38/363). See also Al-Tawqif `ala Mahmat al-Ta`rif by al-Munawi, p. 202. Ibn `Uthaymin said: “Buying and selling are intrinsically linked…It is well-known that there is no selling without buying and no buying without selling. They are an example of two antonymous terms with synonymous meanings, which is a feature of the Arabic language. Most use the term “bay`” meaning “to sell”, from the root “ba`,” which is the range of one’s outstretched arms. [2] Tahdhib al-Lughah by al-Azhari (3/150, 151); Al-Mu??ali` `ala Alfaz al-Muqni` by al-Ba`l?, p. 270; and Lisan al-`Arab by Ibn Manzur (8/21, 23). See also Al-Mughni by Ibn Qudamah (3/480).

Technically: It is the exchange of wealth,  [3] i.e., handing over and taking its equivalent, and this can only take place between two or more people. This can only happen with things that are unconditionally permissible to acquire and benefit from, so things such as pork, intoxicants, impure carrion, and vermin. even if it is an upfront payment for a deferred good (fi dhimmah) [4] This includes sale of an abstractly defined good, such as selling a garment of such-and-such description. or permissible service [5] For example, selling the usufructuary right of passage through one’s land for its equivalent value. equivalent to one of them and in perpetuity, [6] This excludes leasing and borrowing. not including usury or a loan.  [7] Usury and loans are excluded from the definition of trade. Usury is impermissible, and loans are not considered trade. What is intended here are benevolent loans. See Zad al-Mustaqni` by al-Hajawi, p. 100; Kashshaf al-Qina` by al-Bahuti (3/146); and Al-Insaf by al-Mardawi (4/188).

Others said: It is the exchange of wealth for wealth in a specific manner. [8] This excludes usury and loans. See `Umdat al-Fiqh by Ibn Qudamah, p. 53; Al-Bahr al-Ra’iq by Ibn Nujaym (5/277); and Hashiyat al-Bajirimi `ala al-Khatib (3/4).