I only sold phones to those people sitting on folding chairs. Some of the phones I sold were faulty, but did work for periods of time. At the time they check, if it works and can switch on, then they will offer a price. But warning you, that sometimes you can be buying a faulty phone because of people like me.
Once had a guy instantly snap up my phone. I confirmed the deal selling at HK$300 and the customer approached and brought it for HK$450. That one was working properly! Just wanted to buy another mobile then.
I also would not recommend you do it without going through that middle man. Since they have friends on the lookout to stop 'unlicensed' transactions on that street.
6300, try the Sin Tat Shopping centre, but don't hold too much hope. Else those street guys may give you HK$50 for the parts.
Are you an overseas Chinese? You here for holiday or work?