Here’s an important reason why it’s a really bad idea to get a dog from an online market place:

The story is from the UK where the situation is probably much worse than in the Netherlands, but the underlying problem is the same that puppy mills use fertility hormones to get bitches to produce litters continuously.
In contrast, in the Netherlands a breeder registered with Raad van Beheer is ONLY allowed to have 1 litter per year (and they personally check every single home when litters are announced to make sure the house is safe for puppies and the bitch). There are also checks on health of the parents.
Many people think adoption is the answer but it’s not – puppy mill dogs often end up in rehoming for a couple of reasons.
First, they typically have serious behavioural and health issues due to their terrible start in life – stressed mothers create anxious puppies, and they are poorly socialised. No health checks means hereditary illnesses, giardia, etc.
Second, sellers of puppy mill dogs do not care where the dogs end up – there’s no vetting of suitability of homes nor if the new owners are prepared for the breed they’re getting. A responsible, conscientious breeder makes sure their dogs end up in good homes, and most importantly they will ALWAYS take the dog back if the new home decides to surrender the dog – they are partly responsible for the puppies throughout their lifetimes.
It’s an ecosystem of horrible, and we need to understand the full picture – the ONLY reason there are dogs for adoption is irresponsible, reckless human beings. Just adopting dogs is not the answer – to really, truly improve the wellbeing of dogs we need to start being responsible in other parts of the ecosystem too.
(And yes, humans have really behaved badly in many places when it comes to selective breeding of dogs – brachycephalic dogs being a good example of that. Now that we know better, we need to do better and clean up the mess we’ve made to create a future where dog wellbeing and health is the priority.)