"They have rolls of "Nylon Taffeta", is this waterproof or water resistant?"
Typically, the local sewing shop does not specialize in outdoor fabrics and the sales clerks don't know too much about different nylons. Even if they tell you something, it is not guaranteed to be accurate.
There are some nylon taffetas that are most certainly not waterproof or water resistant. Others are, to some degree. Without testing it, you may never know. That's why most of us here who purchase outdoor fabrics tend to purchase them from an online specialty store that knows them and describes them well. The price may be higher or lower, but at least you will know what you are getting.
As a general rule, any taffeta that you purchase locally will not be the best or lightest. However, if your purpose is fairly ordinary, the local taffeta may work.
One local store had "outdoor nylon" and described no further. I purchased one yard, took it home, tested it for waterproofness, and then returned and purchased another ten yards of it. It was good enough, but if I had to do it again, I would likely purchase it online.