A plain resultative names any outcome state (看懂 understood, 寫好 done well); 掉 names one specific outcome — the object is gone. 掉 is a resultative narrowed to disposal.
把 fronts the definite object before the verb (把鞋賣掉); 掉 is the result glued to the verb. 把 sets up the disposal, 掉 delivers it; they often ride together.
我掉賣了那雙鞋 ✗ → 我賣掉了那雙鞋 ✓ (掉 follows the verb, never leads it)
我看掉了那本書 (for 'finished reading') ✗ → 我看完了那本書 ✓ (reading does not make the book vanish; use 完 when the object stays)
我吃掉一半 ✗ → 我吃掉了一半 ✓ (when 了 and the object both follow 掉, 了 sits right after 掉)
English marks this with a particle after the verb — sell off, eat up, throw away. Chinese fixes 掉 in that same slot, and the object follows 掉 with no preposition.