When sally realized her vintage clothing pieces were being bought by the local college students and not just their parents, she started adding more vintage pieces to her inventory. Sally is?

I believe the answer is: recognizing a hot trend and riding the wave.

Trend refers to the general direction where something that would become mainstream is heading. Riding the wave refers to taking advantage of such trends in order to obtain a certain level of benefit (in the case above, the benefit would be financial)