I would haggle if it is second hand stuff. So I would haggle if it was a car boot sale (yard sale). I try to think of what I want to pay then I know when they name their price if it is within haggling distance. Too high and I would walk away, and as low or lower than my price I would just buy. I wouldn’t haggle if it was new stuff on a market stall or in a charity shop though. It’s for charity!
Last year I haggled in Emmaus – a charity shop in France – because my friend told me it was expected. They wanted 60 Euros for a table and I thought that if he reduced it to 50 that I would take that. He said 40, so that was great and we bought it. All this in French too! (“C’est un peu cher”)
So I guess my point is that I would try to find out it it was expected or not. I wouldn’t just have a go.