This is a really good review on what issues might arise with puppies who have not had a great start in life – the focus is on pet stores (which are common in the US) but you could substitute it with a puppy mill or a puppy/dog that has been rescued off the street or from terrible circumstances. If you adopted, you may or may not experience these issues so it might be worth reading this because it might contextualize your dog’s behaviour.
Adopting is great, but IF it’s not the right choice for someone, for whatever reason, the best thing to do is to find a responsible breeder through the Dutch kennel club. That guarantees that the living standards are regulated and checked by one of their staff, puppies are DNA tested and depending on breed health tests are also required. After that, it’s the buyer’s responsibility to interview each breeder for their ethos and approach: e.g. what early stimulation and socialisation they do (these questions are available if you Google what to ask a dog breeder).
Crucially, a breeder should ALSO ask YOU a lot of questions – if they do not ask about your living circumstances, your knowledge of the breed etc. move on and keep looking because that person does not relate to the puppies as living creatures with the right kind of responsibility and respect.
Unfortunately, there can still be still people like this in registered breeders so if you are going to buy, it’s worth doing the process extremely well. A large part of the cost of a well bred puppy is for health tests for parents and compulsory vet/kennel club fees, and the rest is you paying for someone spending hundreds of hours on laying the crucial groundwork for the dog you’ll share your life with for 10-15 years – skimp here and you may spend the same money on a behaviourist later.
It may take a while to find the right person, you may need to wait and it will probably be more expensive than a puppy from Marktplaats but as you can see from this article, the risks of not doing this process properly are significant (not to mention the ethical questions!).
Obligatory caveat: there are no guarantees, dogs are not robots and genetics/breeding is not an exact science. But you can stack the odds in your favour to have a wonderful, long life with a dog. Wanting a balanced, stable dog is not a bad thing – they all need training and work. In the process, we might go some way to cut down this unethical business for dogs.