Middle English sale, from late Old English sala "a sale, act of selling," which according to OED probably is from a Scandinavian source such as Old Norse sala "sale," but in either case from Proto-Germanic *salo (source also of Old High German sala, Swedish salu, Danish salg), from PIE root *sal- (3) "to grasp, take."
The specific application to a public auction is by 1670s. The sense of "a selling of shop goods at lower prices than usual" is attested by 1866. To be for sale "available for purchase, intended to be sold" is by 1610s; on sale in the same sense is by 1540s; the earlier form was to sale (late 14c.). Salariat "the salaried class" is by 1918, from French. Also see sales.
"of or pertaining to sale, sales, or the business of selling," word-forming element from genitive of sale (n.), by 1520s, in salesman. Cf. saleswork "work done for sale" (1775). For earlier use of similar formations, compare craftsman, oarsman, both Middle English. Sales tax is attested by 1886; sales clerk by 1863; sales associate by 1946. Sales representative is from 1910.
"public sale, auction," 1680s, from Dutch vendu, from obsolete French vendue "sale, selling price," from vendre "to sell," from Latin vendere (see vend).
1650s, "capable of being obtained for a price; that can be corrupted;" 1660s, "offered for sale," from French vénal, Old French venel "for sale" (of prostitutes, etc.; 12c.), from Latin venalis "for sale, to be sold; capable of being bribed," from venum (nominative *venus) "for sale," from PIE root *wes- (1) "to buy, sell" (source also of Sanskrit vasnah "purchase money," vasnam "reward," vasnayati "he bargains, haggles;" Greek onos "price paid, purchase," oneisthai "to buy"). Typically with a bad sense of "ready to sell one's services or influence for money and from sordid motives; to be bought basely or meanly."
"public sale in which each bidder offers more than the previous bid," 1590s, from Latin auctionem (nominative auctio) "a sale by increasing bids, public sale," noun of action from past-participle stem of augere "to increase" (from PIE root *aug- (1) "to increase"). In northern England and Scotland, called a roup. In the U.S., something is sold at auction; in England, by auction.
also o.b.o., abbreviation of or best offer, by 1969 in for-sale classified ads.