Just a quick soapbox.....I would not give him money for an accidental litter unless he is using the money to spay and neuter his current dogs.
Its not about spending big money...its about either giving money to a responsible breeder who health tests, etc or about giving money to rescue. Yes, accidents do happen (though they really shouldnt if one spays and neuters) and those pups are of course deserving of good homes. The owner of the pups best course of action would be to give the litter to a rescue to properly screen the homes, require spay/neuter contracts, etc. and then to spay and neuter his current dogs.
Off soap box
I am not an expert on puppy picking.
There are a bunch of articles on it but in general its going to depend a bit on what you are looking for. If you are looking for a dog to do a lot of work, agility, flyball, etc its going to be different from looking for a family dog (though they both definitely overlap!!!)
Look how the pups react...will the come when you call (like clapping etc, not a true recall) are they wagging and friendly? how do they react with toys? dragging a string? How does the pup react when separated from the litter. Another thing is how the pup reacts to loud noises, touch, etc. those are some things to look at.
I wouldnt go for the pup that is crazily chasing and playing as for me, that may be too high drive and energy. I also would not go for any shy dog that hangs out in the back and has to be really coaxed to you. If they are barking is it happy bark or scared?
Those are just some things to look at. If you could swing it, it would be great to have a trainer or behaviorist...or a friend very experienced with dogs go with you.