Being popular is not always good for a breed. Popular breeds like Chihuahuas tend to be desirable target for puppy farming.
A puppy farm is a breeder who produces large quantities of puppies every year. With little care for the animals health or welfare. Puppies produced this way are far more likely to have health or temperament issues.
If an advertiser is posting lots of adverts it is likely to be a puppy farm. If the puppies are over priced this is another bad sign. All adverts in the highlighted area are likely to be from unethical breeders or puppy farms.