卖出 is a resultative verb compound (r.v.): 卖 (to sell) + 出 (out/completion). It emphasizes that a sale actually happens or is completed, unlike plain 卖 which just means 'to sell' as an action. In contexts like stock trading or retail, 卖出 often implies 'sell off' or 'sell successfully'. You can also extend it with 得/不 for potential complements (卖得出 'can sell', 卖不出去 'cannot sell').