I think things have changed mainly because big companies can afford loss leaders and bigger adverts than the smaller outlets. I can remember when I bought my first hi fi, I had to go to a small shop, Then the big manufacturers came in seeing there was money to be made and persuaded big shops to take their products. You went to the small shop got all the listening done, and then went around the corner to a box shifter and bought it 10% cheaper, and then, oh dear, the small retailer went down, but the box shifter did not of course. Now its very hard to find a real hi fi or indeed any tech product seller since the likes of PC World-Curry's etc just do it sale or return, just like Amazon do, yes the online addition has meant you cannot find as many box shifters either. As for cartels, they are often run by the makers these days, who seem to have preferred outlets. I mean if they won't supply you unless you agree certain terms, it does not matter if you are a Tesco or jo bloggs foods.
Brian