I sold a puppy that I had bought from a Breeder, now I'm being sued

Hello, I purchased 2 puppies from a Breeder (both with health certificates) and within a week realized 2 was too much for me. I then sold one of the puppies to another family and have them the health certificate. Within a week they contacted me saying the puppy started acting lethargic so they took it to the vet. Turns out it had a liver shunt so they decided to put him down. They then contacted the Breeder and from what I understand, he gave them a new puppy. They are now coming after me saying they want to be reimbursed for the vet bills. If not they will be be coming after we with their lawyer. Is there anything they can legally do? Neither the Breeder nor I were aware of any health conditions this puppy had, and from what I understand they already received a new puppy. Also, we had no contract, it was just a good old fashioned cash deal. Thank you!