As a matter of interest, both Chambers and Roget's thesaur(i/uses) list the words as synonyms of each other. Maybe that's why?
I do see your point, however, Whickerman.)
Thanks quizmonster, wasn't aware of that. Though I still can't bring myself to 'barter' over a price - swap you some chickens for that toaster? - I would happily haggle.
When i was in school, to haggle meant to argue over a price until agreed, whereas barter meant to swap goods or services in return for other goods or services.
But I suppose language changes over the years.