Only makes sense to exchange a pet that way, though. The sale of non-livestock animals is forbidden on Craigslist. The intent is, of course, to ensure that the scourge of the Earth known as pet breeders do not use it as free advertising to make a profit from animals.
However, it is perfectly acceptable to request a fee for the necessary veterinary costs associated with the animal you are rehoming. You’ll very often see ads like these.