Middle English sellen, from Old English sellan "to give (something to someone), furnish, supply, lend; surrender, give up; deliver to; promise," from Proto-Germanic *saljanan "offer up, deliver" (source also of Old Norse selja "to hand over, deliver, sell;" Old Frisian sella, Old High German sellen "to give, hand over, sell;" Gothic saljan "to offer a sacrifice"), ultimately from PIE root *sel- (3) "to take, grasp."
Meaning "to give up for money, accept a price or reward for" had emerged by late Old English, but in Chaucer selle still can mean "to give." Students of Old English learn early that the word they encounter that looks like sell usually means "give." An Old English word for "to sell" was bebycgan, from bycgan "to buy."
The meaning "betray for gain" is from c. 1200. Slang meaning "to swindle" is from 1590s. To sell off "dispose of by sale, sell all of" is by 1700. To sell one's soul "make a contract with the devil," often figurative, is from c. 1570. Sell-by in reference to dates stamped on perishable packaged foods is from 1972. To sell like hot cakes is from 1839. To sell (someone) down the river figuratively is by 1927, probably from or with recollection of slavery days, on notion of sale from the Upper South to the cotton plantations of the Deep South (attested in this literal sense since 1851).
1838, "a hoax or planned deception by which a victim is 'taken in,' " from sell (v.). The sense of "advertising technique" is attested by 1952 in the phrase hard sell.
"merchant, vendor," c. 1200, agent noun from sell (v.). Seller's market, in which demand predominates, is recorded by 1934.
"action of selling," early 14c., verbal noun from sell (v.). Selling-point is attested from 1953 as "retail outlet;" by 1959 as "persuasive fact in a sales pitch."
also sell-out, "corrupt bargain," 1862 (in Mary Chesnut's diary), from the verbal phrase (which from 1796 meant "dispose entirely of one's interests" in a company, etc.); see sell (v.) + out (adv.). Meaning "event for which all tickets have been sold" is attested from 1923. The verbal phrase sell out in the sense of "prostitute one's ideals or talents" is attested from 1888 (selling out).