Manufacturers collude differently depending on how they sell their products. When they sell through the same retailer, they purposely set prices higher to get more profit from the retailer, even though it lowers overall industry profits. But when they sell through different retailers, it's easier for them to collude. However, having one retailer for all products can lead to lower overall welfare because that retailer can control the market. Also, making contract offers visible to rival retailers doesn't always help with collusion.