I have gone to expos and come home with mites but no new critters. I wouldn't point any fingers at the seller, especially if you handled other snakes.

I treat every new snake as if it has mites no matter what the source. I'd rather be pleasantly surprised not to find any than get a nasty shock a few days later. This means that even before the snake comes into the house the enclosure, hides, paper for substrate, etc. has been pre-treated with PAM/NIX/RID/Permethrin, making QT an inhospitable environment for mites. Also, before putting the new arrival into its QT enclosure it gets wiped down with a white paper towel moistened with Reptile Spray; that's the first thing I do when it comes out of the box if it's been shipped. If the snake has mites the little so-and-so's likely will show up on the paper towel.