For the most part, legit breeders don't post on CL.

If someone is looking for a stud for their ACA registered XYZ, it's probably a BYB.

"accidental " litters of purebreds? Most likely BYB.

If you read a bunch of them, you get a feel for the legit rehomings and a BYB or pet flipper (yes, people do take "free to a good home" pets and then sell them)