When choosing a new snake, you need to pay close attention, don't let them rush you: watch the snake move, look for odd movements or caved in areas (can be broken ribs);
Look closely at the eyes (any stuck eye-caps or other damage? look for tiny mites also, sometimes around the eyes or under the chin, in the water bowl too); let the snake gently & loosely crawl thru your hands, & with very light restraint, hold the snakes face near your ear & listen to it's breathing; ask them to open the snakes mouth gently & look for signs of infection or bubbling/mucus; look at the snake's tail- the vent area should not have 'residue'. Pay close attention to the snake's tongue-flicking too: I once got a snake -a ball python- shipped to me whose tongue-tips were stuck together, & he also had a small swelling (the same infection) in his upper throat. The tongue tips should be separate, & flicked at normal speed. (<if you're new to snakes, that might not be so obvious). Overall, the snake should have good strong muscle tone when held, and be able to easily right himself if place on his back (gently in his cage). He should not be listless. Look for head wobbles** or anything else that doesn't seem right. (**this is something you see in some B.P.s, but personally not acceptable to me)
And by the way, you said the store "had tons of them": another reason to run elsewhere, IMO. It's just asking for trouble. (it's similar to the way that colds, flu, measles & head lice circulate thru a kindergarten class, & again, remember that stress, whether in humans or animals, reduces the immune system's ability to function.)
I "second" the advice from bcr229 also: do your own research first & set up the cage ahead of bringing home a snake, to make sure you have the right temperatures & humidity, etc. Most pet stores are there to make sales, very few sell all the right equipment, and fewer still give reliable care information. Better luck next time, "practice makes perfect".
