This is the place if you are looking for a large mount of currency exchange. The address is 解放路166号 ( Jie fang lu 166 )  Here is the coordinates

So how to spot those people who do the black market business? Most likely they sit on the couch, look around, look like they are expecting someone.  They usually carry a big bag with them.

Another concern is security issue. In my opinion, it won’t be a problem. They have CCTV in every corner. Some of them i knew have been doing this business for a long time. It’s a business based on reputation. So basically it’s a great way for currency exchange.  Just be careful with those bills. They might have fake bills mixed with other real bills.