Good point, Amanda -- it works both ways, and the person getting the puppy should see certain things from the breeder.
Alex was 8 months old when I got him, I wasn't picking a puppy from a litter, but Linda had several males she was going to place -- pulled from the show ring at various stages in their show careers, for various reasons; Alex because he was clearly not going to get to the 20" height that is optimal in the standard -- and Linda knows me and knows the kinds of things I do with my dogs, so she told me Alex would suit me best, because of his enthusiasm for new places and activities. (In retrospect, I think she was saying he was a brat. LOL!) She was right, and I allowed her to choose the dog she would place with me.
The breeder should be pretty good at assessing temperaments and personalities even at 8-10 weeks, and should know the parents and the pedigree well enough to make a prediction about how a particular pup will mature. Matching that to the applicants is what a good breeder does.