If the pet store is one of few that do know how to care for herps, enough to feel comfortable buying from, mites should not be present. Mites are about this size . and can be hard to see on some snakes if the problem is yet bad. Takie a white towel and have the snake crawl through. If you see what looks like specks of pepper, it probably has mites.
Look for a snake that flicks it's tongue when exploring or checking you out. Look for eye-caps, skin at the tip of the tail, or patches of skin, which would indicate poor shedding. Visually and with a towel look for mites. Look around the facial pits and eyes for mites. Look at the cloaca region for swelling or crusted waste. Snake should not be thin and backbone not pronounced, Feel for lumps or skin abnormalities and look for any other skin problem. Belly should be firm.
Best of luck