Is it farther or further?
late Old English sala "a sale, act of selling," from a Scandinavian source such as Old Norse sala "sale," from Proto-Germanic *salo (cf. Old High German sala, Swedish salu, Danish salg), from PIE root *sal- (3) "to grasp, take." Sense of "a selling of shop goods at lower prices than usual" first appeared 1866. Sales tax attested by 1886. Sales associate by 1946. Sales representative is from 1910.