I'm not saying they're perfect (I've on two occasions spotted goods beyond the 'use by' date in their food hall which I discreetly drew a member of staff's attention to), but it was M&S's store in Princes Street. It definitely had to do with at least one of the items of clothing. I couldn't hear the exact conversation but from the way the pair kept holding the items up and closely inspecting them whilst at the same time remonstrating with the assistants, I reckon that they had either spotted flaws or, what in retrospect seems more likely, were actually trying to find flaws in the hope of getting a price reduction - so, yes, they could have been haggling!
I'm not into haggling myself, but I have chanced my luck if goods are slightly but not noticeably faulty. In fact I recently bought a small lamp which has a little scratch on its metal base and got it for £11 instead of £22, with no argument.