Change the sentence into indirect speech.
He said, "I bought this laptop a few days ago."

The correct statement is "He said that he had bought that laptop a few days before." 

As a rule, the simple past in the direct becomes past perfect in the indirect. The pronoun 'this' becomes 'that' and words that are expressing nearness in time or place are changed to words that express distance thus, 'ago' becomes 'before'.