What does "it" refer to here?
Native speaker’s answer
Rebecca
In the previous couple of sentences, the speaker introduces the idea that the young generations are driving the fashion reselling trend. She states that there are three main reasons, then introduces the first reason: "the first is our desire for variety ..." The "it" that follows is related to this idea. Thus, we can understand "it" to mean the fashion reselling trend. Our desire for variety is one of the reasons why fashion reselling is growing.