I'd take the job. You'd be surrounded by friends & animals - what could be better? The only way to know the extent of how much the puppy-thng would bother you is to work there. You could do some investigative work and find out if the puppues really do come from reputable breeders and educate many potential pet owners.

If you work there for a month and find that you are having a really hard time with the puppy issue, then you can leave on a friendly basis - - claim your schoolwork was suffering or something. Either way, you'll have earned some extra cash AND learned some useful information about the animals in a pet store.