I would just stick it out with treating the snake. He/she is probably better off with anyone other than the previous owner. It would just be more of a hassle(IMO) to get your money back, than just to treat the snake and be done with it.

Provent-A-Mite works through chemical innervation(meaning it chemically kills the mites). You must NOT get any of it on the snake itself, just follow the instructions on the spray canister when you get it, and it should be very clear on how to go about it. You're going to have to use newspaper for substrate or white paper towels so you can easily see the mites, and the mites won't have much to live in. Clean the cage everyday, change the substrate and clean the hides and waterbowl. A bleach water solution is a good idea. I would use the plastic planter saucers from WalMart as hides for now(they are good hides at anytime regardless), but just for the ease of cleaning.

Maybe someone a little more experienced can explain how it kills the mites on the snake. I have never personally used the Provent-A-Mite as I have never had a need for it, but hopefully someone who has can help.