What is the meaning of 이거다 싶어서 고민 안 하고 샀어. I don't understand why it's 다 after 이거 so I don't get the menacing of the sentence. I know 다/가/나싶다 means

What is the meaning of 이거다 싶어서 고민 안 하고 샀어. I don't understand why it's 다 after 이거 so I don't get the menacing of the sentence. I know 다/가/나싶다 means