把 O V 了 keeps the subject as the doer disposing of the thing; 被 flips the spotlight so the thing is the one acted upon. Same event, opposite agent in front.
V了 alone just completes the verb; 把 O V 了 first fronts a definite object and demands the verb dispose of it. Without 把 there is no fronted-object disposal frame.
✗ 我把一本書賣了。 → ✓ 我把那本書賣了。 (把 needs a definite object, not 'a book')
✗ 我把它喜歡了。 → ✓ 我把它丟了。 (the verb must dispose of the object; 喜歡 acts on nothing)
✗ 我把它賣。 → ✓ 我把它賣了。 (the disposal needs 了 to read as done)
English keeps the object after the verb ('I sold it'), so learners drop 把 and front nothing. Chinese pulls the definite object ahead of the verb and seals the result with 了.