
it is possible that they were removed, but if the root of the claw is left behind they can grow back, when they do grow back they can often be mis-shapen too. maybe that is why you only noticed it now as they were all removed at the time and take a good few weeks to grow back, just a possibility. i have just had mine done (though i do them at 1 day old & only the rear ones too). i checked and double and triple checked them when i got home.
As this is the first time it is all new to you so as you say you would expect the vet to do his job correctly, just learn from it and in the future always check them when you get home.
just ensure that the buyers know if their chosen puppy is one with dew claws remaining. :)