I think it's okay to specialize in one sort of fabric/line - I think I would rather have a fairly complete selection of one type - example - civil war - and maybe skip the 30's reproductions - than 6 of one and 6 of another - it would be difficult (and expensive) to carry everything of every line. (or vice versa)
If people sign up for classes, be nice to the student that bought his/her fabric from somewhere else.
Be nice to everyone - the big spenders and the "just lookers" - the "just lookers" might have big spending acquaintances.