Am I obligated to refund the money to a buyer who agreed to purchase the item after being informed of its defect, but later claimed he did not focus on the details of the defect and demanded his money back?

If the seller informs the buyer of a defect in the merchandise and the buyer accepts it, the purchase becomes binding, and the buyer has no option to return it or claim a reduction due to the defect. Acceptance of the defect precludes rescission and claiming a reduction.