I Think I Bought a Sick Snake

So last week I picked up my first corn snake at Repticon on 3/16. He seemed fine when I initially checked him out at the convention, he's looks like he's a good weight, is curious and alert and he flicks his tongue with no problem. I took him home and left him alone in his terrarium for three days before I held him again. When I held and inspected him for the second time, I noticed I could hear him breathing - each breath he takes is wheezy sounding. Today it seems to have gotten worse, his breaths are a little more forceful, almost sneeze like. Could he have gotten sick this fast, or was it more likely that he was ill when I bought him? What do you do if you buy a sick snake from a breeder? I know he needs medical attention, but I was wondering if it's customary to return sick specimens to the person you purchase them from? He's eaten once, pooped, drank water and seems alert, but his breathing is so audible and I can feel him taking labored breaths when I hold him. I don't have any problem taking him to the vet but I just wanted to see if it was common practice to return sick snakes.