I'm not from HK, but on my trip there last year, I believe that there are quite a number of shops that sell used bags, whether they actually have auth Bbags depends on their stock. Milan station and Paris station (if my memory serves me correctly) has a few branches each. You can check with your hotel conciage for their addresses and which one is nearer to you.
There is a balenciaga boutique in central right before the slope up Lan Kwai Fong.
Joyce boutique in TST carries quite a number of B-bags there too.
THere's also Milan stations that have brand new and used balenciaga. The Milan at Lan Kwai Fong has rouge vif and the rouille city brand new last week. You can try the Milan at causeway bay too. THey have some good cities sometimes.
In places like Milan station, you'll find a few, it all depends. Some stores have more than others. But the plus side is in places such as Causeway Bay, there are many of these resellers shops within walking distance of one another. There's Milan Station, Vip Station, Paris Station (haha I know their names are quite funny). But in these shops, you'll find mostly LV and Hermes.
If you have any local friends who has something called a Joyce card, bring them along with you to Joyce or the Balenciaga store in Central. You'll get 10% off normal merchandise including bbags.