There are some issues with the place it sounds like.

It seems to me they told you the breeder gave them sick snakes that were mite infested. It sounds to me that the store is the one with the problems. As a professional in this area they should be able to see and treat most issues with their animals. Your story tells me that they either don't know how or just don't care. I will say that if a place gave me one snake with mites I would bet any other snake they gave me would also have them. They are going to need some deep cleaning to get rid of the lil pains.

Also most any snake with mites will stop eating. If the new snake has signs of dead mites I would probably treat the snake as well in a couple days. What did you use to treat the mites on the first snake?

Also with the new snake you are going to want to provide a perfect climate for it. If their is a chance it has an RI and/or mites it is going to have problems and providing a perfect climate will give it the best chance to get better.

If you would like we could go over your set up and make sure it is all on point.

I will also add that when I bought my GTP it had a RI that the owner must have known about and didn't disclose. I took care of it and got the full workup to get him better. One thing they had me do was raise the temp and humidity while I was treating it. Having been through that if I had to treat another snake I would follow the same steps and raise my BPs hotspot to 92 and keep the humidity bumped a bit.

If this snake doesn't get better going straight to the source is your best option. Take out this middle man. I wouldn't ever go back either. You can bring home mites unknowingly so if I know a place has a history of mites I would steer clear.

In addition if you do get another snake you are going to need a deep cleaning for all your snake gear too. May not be bad to do one anyway. Who knows what the store sent you home with.