I would guess the dog is around a lot of other dogs or people that don't pay much attention to it. Dogs are pack animals and puppies try to find their place in the pack. Many of them do this by biting. If you take the dog the first thing you need to teach him is that you are in charge. You are the pack leader. Chances are the biting will stop then. It is doing this that can be the challenge.
If it were me and I really wanted the dog I would contact a trainer, tell them what is going on and see how hard this is to break. The trainer will be straight with you and that should give you some good info to make your decision with.
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