A copy of the dog’s medical records, indicating vaccinations.
A couple days’ worth of the food they were feeding
Contact information (ideally email and phone)
Specific instructions about care, treatments, etc.
A towel or item that has the scent of the litter and mother
Asking questions will make you feel more comfortable about welcoming a new pet into your home. Proper and responsible breeding, appropriate healthcare, and socialization will make a big difference in how healthy your puppy is and how happy they are with their new homes.