I would suggest never buying a puppy that's been separated from its mother too young. Whilst it may sound harsh, if you pay a 'breeder' who mistreats their animals, what you're doing is encouraging them to do it again.
However, I also know - it's not always that easy. Dogs do need somewhere to live. Even the ones who are 'damaged goods'.
I'll point out this - normally puppies are separated from their mother at 8-12 weeks. The first few months of a puppy's life are called the formative period - it's worth your time looking that up and having a bit of a read, because it's incredibly important. It's the time during which your puppy forms associations and learns 'the world'.
But up to about 8 weeks, their mother is doing all sorts of important teaching on the subject of 'how to be a dog'.
There's quite good evidence to suggest that dogs removed early have 'problems' for the rest of their lives.
- http://www.petsadviser.com/behaviors/severe-separation-anxiety-in-puppies-at-night/
- http://www.markvette.com/the-formative-period/
- http://pets.webmd.com/dogs/news/20110825/puppies-litters-behavior-problems
So if AT ALL possible, you should take that puppy back to its mother, because it's very important for the rest of it's life.
If you can't, then you need to accept that by adopting that puppy, you're setting yourself up for a hard time. It may well have behavioural problems and be generally 'hard work'. Kudos to you if you do, but ... well, it'll be hard work.
Puppies do graduate from 'being puppies' - about 8-12 weeks, leaving mum is always a little difficult, but not really damaging any more. They're ready to form new bonds and make a new 'pack', and so at that point is when you need to socialise your puppy - teach it what the world looks like. It's likely that a puppy introduced to things up to about 4 months won't have a problem with them in future. E.g. take them to meet other dogs, livestock, and most especially humans (adults and children).
I would reiterate my initial statement - it is my opinion that you should never buy a puppy from a shop. You should always make a point of meeting the mother and observing living conditions. A reputable breeder will make some effort that you understand what's involved and how to look after the dog, and they'll also make sure that their dogs are well socialised early.
From a shop... it's a complete gamble, and more likely ... they've been 'farmed' because it's cheap and the people buying don't care.
Actually, I'd strongly suggest if you're looking at getting a dog - visit a rescue and adopt one. There's always plenty of dogs there, through no fault of their own (some are 'problem' dogs, but most have a good history and are only there because their owners died, or got divorced, or got thrown out by their landlord).
