
I would never let anyone see puppies of mine if they had been to someone else's litter on the same day. Not because I think they should have to have one of mine, but as others have written on cross contamination reasons. They could have been to another breeder, whose puppies have Campylobacter from poor hygiene, as a puppy that come to our pet training club had when it was picked up at 5 weeks !
I've actually never sold a puppy via an advert & the prospective owners come via word of mouth & recommendation.
I have no problem with people viewing more than one litter if they are looking for a certain type/temperament etc, but they should, no must, be up front with all the breeders they go to visit about this. Sadly from my friends experiences this is rarely the case, I've have friends who had prospective owners who have been to see the puppies regularly & booked a puppy, only to ring up the day they are supposed to pick the puppy up to say they've got one from elsewhere or changed their minds-fine it's their right, but not to tell the breeder until the day they are to collect is a big no no for me.
The last Border Collie I bought was selected by me at a day old from a photo & I never changed my mind over the next 6 weeks, I did visit him several times before picking him up & as I chipped & pinned the litter as well it was almost 1 a week from when their eyes opened.
Roodee(Cavalier)was a litter of two & we never met him or his breeder & dogs until the day we picked him up, but we had been in regular contact from before he was born & he is everything we hoped he would be. His breeder & I do have a friend in common, which was how I found out about him.