Mites aren't always indicative of bad conditions or sick animals. I had a mite outbreak once (ugh) started by a BP that was not properly quarantines (needless to say, I've learned my lesson!) I noticed it not long after the arrival of the BP, and none of my animals were the worse for wear. I'd not have deemed them sick or in bad conditions at the time. It's like getting, lice really. It happens.
Assuming this is the same snake, and she got it from the reptile store, then i'd not be in the least surprised to hear that it has mites. petstores are heavens for those things.
At any rate, there's not much that can be done by contacting the dealer, except possibly notifying him of a situation he was not aware of prior.
Goodluck getting rid of the mites... follow the instructions on my site and it'll work great
Zoe