I think your biggest concern should be not where to buy bike, but how to register it to remain legal as much as possible.

all bikes I listed (2 Jialings and 1 Qingqi) we bought from dealers in Xining (Qinghai) and Shanghai areas. and luckily there's no problem to remain legal in Xining as this city is not banned for motorcycles.

for 2nd hand bikes you could use also 2.taobao.com
but if you're willing to buy 2nd hand then you have to go and check bike carefuly yourself first. no any downpayment ahead (I had a very unpleasant experience with one 'respected' MCM member).