YOu will find as I have about 90% of the general public will go on your recommendation.
Have a good house strings, syn, Multi, and poly and then carrry a few popular strings.
For syn.. you can't go wrong with Prince syngut with duraflex.
Multi Livewire is inexpensive and easy to sell..
poly. what ever, you like.
then carry some others like X1biphase, ProHurricane, maybe a couple of textured poly's.
That will suit probably 95 to 99 percent of your walk ins. and by walk ins.. I'm not including your friends who have a particular likes or dislikes.