I am not so sure where you can find cheap gear. However, my favorite place to go to is Jaben. I find that in Jaben there is plenty to choose from and the space is big and comfortable, it has a good atmosphere. The staff are also very nice and kind. As for second hand, I am really not sure, it might be that it is more popular in Japan for example e earphone store in Osaka. Hope that helps!
Forgive me Nicolas I misread your question, I thought you meant second hand shop/store in HK. I rarely shop online for earphones but I think you can trade and buy second hand in this forum. Sorry if I was not much of a big help to you.