The -고 verb is what happens before the selling: 쌓아놓고 팔다 "stack them up and sell", 깔아놓고 팔다 "lay it all out and sell", both describing how a market stall displays its goods. The most useful item here is 사고팔다 "buy and sell", which three lines use as a single fixed pair for trading — 나라로 물건을 사고 팔고, trading goods between countries. 팔다 is a person selling; 팔리다 is a thing selling.