17. If you hope to buy the ideal goods, you should shop around.
想买到理想中的商品，就需要货比三家。
还能这样说：
You should inquire the goods at other shops, if you want the dreamed goods.
Shopping around can make you get your ideal goods.
18. She always purchased the goods in good quality and low price.
她总是能买到物美价廉的商品。
还能这样说：
She always bought goods of high quality and resonable price.
The goods she bought were usually cheap and in fine quality.
应用解析：
put a price on sb. 悬赏缉拿某人；
set at no price 看得不值钱，毫不器重
19. You'd better buy food in supermarket, for the food there is genuine.
买食品类商品最好到超市，那里货真价实。
还能这样说：
You'd better go to the supermarket to buy food which is genuine there.
You'd better go to the supermarket to buy food, because the food is genuine there.