Question

barry bought a camcorder when i saw him at the mall yesterday
(onde esta o erro)

Answer 1

oii ta errado pq falta o  past perfect, seria certo assim:
Barry had bought  a camcorder  when I say him at the mall yesterday.
You're welcome! 

Answer 2

Nesse caso, pede-se o uso de past-perfect tense para indicar que é algo que ocorreu no passado mas que houve uma outra ação antecedendo, ou seja: Barry had bought a camcorder when I saw him at the mall yesterday.