Online classifieds just aren't a good way to acquire an animal - there's no regulation or way of checking the animal's welfare.
As with all online classifieds, there is a scam risk - in fairness, PAH do have a warning on the site not to pay money in advance, but where an animal is concerned there's a danger of being guilt-tripped into doing this, or getting carried away if you lose your heart to an animal and want to 'secure' the purchase.
People looking for cats and kittens to use in dog-fighting trawl these sites for free or very cheap animals, so I would never in a million years use them to rehome a pet.
There's no way of checking whether pedigree animals might be farmed; or if they are backyard breeders, whether health checks have been carried out.
If you buy moggie/mongrel kits/pups from the site you are encouraging people to breed them in a world full of unwanted cats and dogs.
Your best bet is to go to a reputable local rescue. If you want a pedigree animal, find a breed club and ask for recommendations for responsibly bred, health tested animals - and be prepared to join a waiting list for a litter.
Sites like this pander to the 'I want it now' culture and make it all too easy to buy an animal with little more than the click of a button, rather than giving proper thought to whether you can cope with the animal in your life (I am not saying this is you, OP, I'm talking about the culture that means these sites thrive).
You might be lucky and get the perfect pet as I am sure there are genuine, albeit naive, people using these sites, but you could end up with a sick or behaviourally challenged animal, or one with a genetic ticking time-bomb such as a heart condition.
My advice is to steer well clear.