Honestly, after a month I don't think you can be 100% sure the mites were there when you bought him. I would not call the breeder to complain. Maybe call to ask if he has had any problems with mites, but I would not accuse him of anything. As for the non feeding issue, it could just be off feed for the winter but many snakes go off feed at between 750 and 1000g for a while. I think it is because almost all captive snakes are overfed especially as hatchlings/subadults and they shut down for a while to keep from getting too overweight. That is just my theory but many snakes do shut down for a while at around that weight whatever the reason. If I were you I would get the PAM to get rid of the mites, and try to feed a small live rat and see if it takes it. I would not worry too much about eating though, at 750g it will be fine for a long time without eating.