26 C.F.R. § 25.2512-8
Transfers reached by the gift tax are not confined to those only which, being without a valuable consideration, accord with the common law concept of gifts, but embrace as well sales, exchanges, and other dispositions of property for a consideration to the extent that the value of the property transferred by the donor exceeds the value in money or money's worth of the consideration given therefor. However, a sale, exchange, or other transfer of property made in the ordinary course of business (a transaction which is bona fide, at arm's length, and free from any donative intent), will be considered as made for an adequate and full consideration in money or money's worth. A consideration not reducible to a value in money or money's worth, as love and affection, promise of marriage, etc., is to be wholly disregarded, and the entire value of the property transferred constitutes the amount of the gift. Similarly, a relinquishment or promised relinquishment of dower or curtesy, or of a statutory estate created in lieu of dower or curtesy, or of other marital rights in the spouse's property or estate, shall not be considered to any extent a consideration "in money or money's worth." See, however, section 2516 and the regulations thereunder with respect to certain transfers incident to a divorce. See also sections 2701, 2702, 2703 and 2704 and the regulations at §§ 25.2701-0 through 25.2704-3 for special rules for valuing transfers of business interests, transfers in trust, and transfers pursuant to options and purchase agreements.
26 C.F.R. §25.2512-8