Most sell either Fathead minnows or Shiners ... but, sometimes Bluntnose or Rosey Red minnows can be had (depending on regional location).
Which variety your man is selling could depend on if he's getting them out of a creek or pond ... and if they're native to your waters or bought from a wholesaler (fish farm).
I "believe" that the Fathead minnow is probably the most used minnow sold through "baitshops" that are supplied by fish farms.