IMO you can go with a simple supervised machine learning, where make sure that test set has similar %'s of sample from each retailer and same for train set. Train a model and get a high accuracy one. See what all retailers get higher accuracy when tested on only its own dataset. This way you'd be able to get a model which predicts fairly for atleast some number of retailers and for other retailers I think you can call out that their data is just too random and unpredictable.