Yes "mites happen"- often at reptile expos, where buyers handling snakes may inadvertently pass mites around from the sellers who brought them to the sellers & buyers that didn't. Pet stores too, often end up with mites, because they often 'buy low & sell high' to make a profit- meaning they're often buying from large wholesalers (plenty of mites to go around there) or from less than stellar sources for their lower prices.
Here's a pretty thorough write-up on mite treatments (from bcr-229)- ask yourself, does this sound like something you want to deal with?
https://ball-pythons.net/forums/show...=1#post2771656
While there's a lot to be said for seeing the snake you're buying in person, & getting hands-on, there's also something to be said for buying from private breeders with excellent reputations (they're out there, ask around, shop around) and getting not only a clean animal (no mites), but one with a feeding record.
That last thing, for a new snake keeper, should be a deal breaker, & the pet store you were just visiting sounds like they're in a hurry to treat & sell, not make sure the snake is even eating. Doing it right (raising snakes from hatchlings/neonates) is not that fast- not if you care & have a reputation to protect. Snakes fresh out of their eggs don't eat that fast, & it's best they eat at least 3 times before they're sold. That likely means they'd be a month old- because they typically only eat once a week. New homes & new owners- are stressful for snakes- & if a snake is not feeding strongly, it can be harder for the new owner to get it to eat at all.
When I raised & sold some snakes, they had rather lengthy feeding records compared to what many others do. Rosy boas, for example- can be a little fussy, much like BPs- & mine all had feeding records of 10 or more meals with no refusals, & all taking f/t. That pretty much ensures that they'll do well for their new owner- it makes a difference.