When you take it to the vet, get copies of all the paperwork, including your bill, and send it to the guy who had the nerve to sell you an ill animal. He is lucky you don't charge him for your time and aggravation, too. Tell him you would like compensation for dealing with an animal that was ill upon arrival. Make sure to get the vet to note if the infection seemed to be long-standing or not.

Be polite and at least civil in your wording. Don't be afraid to phone him, too. However, in the end, if he refuses to handle this situation in a responsible way that leaves you satisfied, rip him a new one on Fauna. If he is unreasonable and/or rude, other people should know. That said, if he does a good job of offering after-care and compensation, mention that too.

Good luck! I hope the lil'guy gets well soon.