I agree to ask around especially the ones that you feel will give you their racquets what they typically hit with, and get those strings.
I would also get PSGD as that is and has been the #1 string sold in speciality shops for many years running as it is a good basic syn. gut for those that have no idea of what they want.
I would also get name brands until your customers know your jobs, as brand recognition goes a long ways with many things.Once you get your clients base up there, and you have their confidence, you can then suggest other strings of lesser known manufacturers that you know they can benefit from.You can then find out what they like or dislike about their strings and make suggestions that may help their game, like hybrids, etc. Don't go overboard in the beginning as you don't want string around that does not move.Get some sets in the beginning, and when a type starts really moving then get reels.