The -어서 verb is what is done to the goods before they go on sale: 만들어서 팔다 "make it themselves and sell it", 섞어서 팔다 "mix them and sell", 희석시켜서 팔다 "water it down and sell it". 만들어서 팔다 is the useful one — it marks a shop as making its own rather than reselling. One line uses the English loan: 가격을 다운해서 팔아버리면, dropping the price and offloading it.