All of this has gotten easier, actually, with the web. It's still a mess sometimes, but using pics always has been.
The TOS explains what is what.
I typically sign up the client with an account. The problem with you buying the pics is after you are out of the picture that the client will take one they like and put it in an ad that goes on the back of Sports Illustrated and, uh oh! That was not the approved use, the client gets sued and then the client sues you for not handling properly. And, if you bought the pics, then they might have a point there.
So, bill the client for "research" or "consulting" on the photo search, but not the pics--and "reselling" them is usually outside of the TOS anyway. Look after yourself, first and foremost. netmeg's story might sound far-fetched to some. It's not.