In my opinion... if someone has infant animals of any sort (any animal under about 3 months or so) they are breeding and using craigslist to sell. This is first of all against craigslist terms and conditions. First of all, why would anyone have babies if they weren't breeding? There are the few cases where people buy a chin and it's already pregnant etc. But if I have dogs, cats, chinchillas, whatever... who are unaltered and mixed genders are living together then I am breeding. It could be out of stupidity, carelessness, or whatever... but it's still breeding in my opinion. You see tons of people doing this on craigslist. It's not really a "rehoming fee" if you're breeding. You intentionally allowed animals to procreate and are now trying to get money for it.
That being said, I agree with the others who say you can't guarantee any kit is healthy. Even if it comes from a great breeding line there is no guarantee it will not have any issues. However, if you have a reputable breeder you can possibly get a refund or at least inform them so they don't continue breeding an animal who is producing sickly kits. Things like fur chewing, malo, and other diseases would be things I'd want to let a breeder know about. Also, much of any animal... or even people, is how well it's cared for after the animal is born. A chin given quality pellets, hay, lots to chew, good environment, etc. is going to have a better chance at being healthy. If you want to take the risk just know it is the risk you're taking. Also, keep in mind you may be supporting an irresponsible breeder. It's not really worse than picking up a chin from Petsmart though... and it sounds like it's a heck of a lot cheaper. Pet stores here want like $180 for a chin and most of them are over 6 months old when you get them. To me if I had no other options I'd rather get an 8 week old for $50 than an 8 month old for $180 if the risk was the same.