구매하다 is more formal way to use (purchase) , 사다 is more informal used. (Buy) So that when u r in office, u more often use구매하다 such as, purchase deptment구매부서. Purchase order (구매주문 =발주서) 사다 is used more commonly. U can replace구매하다 in most of sentences to사다 expcet for sepecific word like used in company 구매부서, 구매수량 등등 Ex) 1. 엄마 과자 사줘 2.부장님이 의자 1,000 개 사놓으래(Same as 구매해놓으래) If u r not clear, feel free to ask more : ) p.s> I don't know why, all sentences i've typed messed up. I've edited again.