If you saw one mite that means there are a ton more you didn't see. They do need a mite killer treatment to fully get rid of them. Cleaning them like you did might make you think they are gone. You got rid of a lot so you might not see them for a while but if only one is left behind they will multiply again and take over with time. There are many reasons snakes can go off food. There are even a few reasons a snake can go off food if not stressed at all. But you did see a mite so those will bother you snake and cause it to not eat. Again because your cleaning go rid of some your snake might start eating again(but not necessarily) but that does mean everything will stay ok. Once the mites bring there levels back up you will have problems again.
If you are really gonna get rid of your snake make sure you properly treat it a couple time for the mites before doing so. Would be a really horrible move to sell someone a snake with mites and have them deal with it when you know you have them.
Some people use PAM to treat mites. I prefer Nix. Some people are against that because its a product for humans. Just make sure you use something that kills eggs as well as the mites. If not the problem would return again if even 2 eggs got left behind.
Well, Are the mites inside the body? Or On the body? Never mind I'm giving it back to the pet shop today.. But thanks man