Bargaining culture splits the world roughly into three zones. In much of South Asia, the Middle East, parts of Africa and Latin America, haggling at markets and street stalls is expected and even enjoyed; the first price quoted to a tourist assumes negotiation. In East and Southeast Asia it varies by venue: tourist markets yes, local food stalls usually no.
Within bargaining cultures, know where the line sits. Souvenirs, clothing, crafts and taxis without meters are fair game. Food, especially cooked food and small grocery purchases, generally is not; the margins are tiny and haggling over someone's lunch prices comes across badly.
The universal etiquette: bargain only if you intend to buy, keep it light and smiling, and once a price is agreed, pay it. Walking away is a legitimate tactic; agreeing and then reneging is rude everywhere. If a shop has printed price tags, that is usually your signal that prices are fixed.