A 'rehoming fee' is how people try to get around the rule of no selling pets. They say a 'rehoming fee' because according to craigslist rules a small fee is allowed to help cover the food and vet care you put into the animal, kind of like an adoption fee like you would get charged at a shelter, but you are not allowed to sell animals, like a breeder selling puppies. Most times when they say 'a rehoming fee applies' in the ad but don't say how much, they're really selling the animal and when you talk to them they'll ask for outrageous amounts, like $300+ which they couldn't just say in the ad because they'd get flagged.
