The seller should be willing to let you take the pet to a vet of your choice for a health check and return the pet within 48 hours for a full refund if there are health issues. If the puppy has already been health checked, make sure you have this documented and phone their vet. The vet may not be able to disclose specific health information due to GDPR, however they should confirm the puppies have been checked. When you meet the puppy, they should be sociable and alert, with bright eyes and no visible health issues. Read about how to check the health of a dog here.
If the breeder claims the animal has been vaccinated and/or microchipped, ask to see records of these, including vaccination certificates and evidence of worming and flea treatments. It is required by law that all dogs over 8 weeks of age are microchipped and that the keepers' details registered against them are kept up to date. The breeder is responsible for microchipping the puppies and must have their details registered on the microchip database first. These records should be sent before collection and you should never accept any excuse for sending them later. For puppies, if they are advertised as Kennel Club registered, make sure you get a copy of their Registration Certificate before you buy the puppy.