I got my last pup from our local “Craigslist” equivalent. She was an “oops” puppy, daddy is an unknown. The local shelter didn’t have any pups when we were shopping, so I was glad to have an alternative information source to find puppies in the area that needed homes. Additional bonus that we picked her out, paid for her, and took her home, with no strings attached, no hoops to jump through, etc.
So no, I don’t think selling animals online should be illegal.