I would suggest any new stringer keep a full reel of half decent syn.gut (eg Prince Duraflex, RAB Texflex, HEad PPS), a full reel of resonable poly (eg Weiss Silver, Signum PPP, MSV) and a half reel of a multi (eg RAB Sensorfibre, Tec NRG etc).
That should sort you out for most regular club players, if anyone want's anything specific you can order an individual set.
Pricewise, set you labour charge and your mark-up on string (if you want). I charge a little more for labour than some of my competitors but that's because I do a better job! I mark up my string based on the 'set' price. That is if I cut from a reel i'll charge the cost per individual set to the client - that's my mark up sorted. You could just charge cost price if you want, it all depends on what your market is and what people are willing to pay.
I tweet - @ashtennis guru (no spaces)
I Shoot - www.flickr.com/photos/ashtennis guru/ (again no spaces! grrr)