The sale of animals through craigslist is prohibited, with the exception of a sufficient adoption fee, if it's to give the pet a new home. However, this method of adoption is very hazardous for the pet, according to articles posted on the subject.
The rules on the craigslist's prohibition list state that animals cannot be sold, animal parts are forbidden from being listed, and animals cannot be put up for stud services. These are usually illegal practices, if not done through official channels, so craigslist forbids these specific sales. Giving a personal pet away for a fee, however, is allowed, but can mean danger for the pet.
According to an article posted by Chicago Now in 2013, craigslist is a place that many people with malicious or illegal intentions check for pets. A pet listed on craigslist is, according to the article, likely to become the victim of abuse, bait in animal fighting, or a research animal for a research facility. Finally, some are likely to resell the pet for a profit, which will circulate the animal back out there and make it more likely to end up in the hands of those who would harm it.
This does not necessarily mean that a pet will end up in an abusive or lethal situation. However, the risk is very real and must be considered when putting a personal pet up on craigslist for adoption.