Can buy single things from whole sale markets?

Yes, you can buy single things from the wholesale markets in a good price.

yes you can buy single items from wholesale markets, wholesale it supposed to be run during night when normal visitors sleep, for clothing I would recommend Beijing ZOO market right next to subway stop just above big bus terminal, reasonable prices and many shops, also many pricetags but always do price research and don't buy in first shop, almost always you will find much cheaper shop few meters away, if not just remember location. another good place for shopping is Xidan, just go to northeastern part nof junction from subway and walk toward north about 300m and market begin there and east from there few meters is also building with market. although best deals you can get on street, like LaCoste T-shirts which cost in small shops around 30RMB, in beijing zoo maybe 20-25RMB you can buy in street stands for 15RMB