I know there's a YANG HUO market in tanggu area of tianjin.there sell lots of Motorcycles.but I'm not sure if they have Harley.

For a classy bike get a ChangJiang 750 sidecar. http://www.beijing-sidecar.com/
Way cooler than a hog.

If you're going to get something in Tianjin, the police don't seem to mind the small mopeds. It also seems that one kind is now allowed, (the ones with the pedals), because there are markets inside the city selling them now. Some of the newer ones are quite quiet and conceal the exhaust pipe, so it looks almost exactly like an electric bike, as well.

I usually go to the market down in tangu every month sometimes more.  You dont see too many harleys down there but when you do they are usually top bikes.  I've seen custom fat boys for around 100k and stock dynas and softails for about 80k.  As far as paperwork goes you can buy anything.  Paperwork, registration and such can be bought, will work in china but probably won't be able to export.  A special liscense plate can be bought, i think it cost around 30k, that plate allows you to drive anywhere in china, and the road tax on a 1000cc or higher bike is around 10k per year. 

If you want a drivers liscense you have to be registered as living in a place where motorcycles are legal, pretty much anywhere outside the thrid ring road of tianjin, and i think some areas in beijing are still ok. 

I ride a vfr400 here in tianjin, if you are careful and know what roads and what times to ride on them the police won't give you trouble, riding without a helmet helps, when they see a laowai they don't seem to care. 

As far as buying a "motorcycle" with pedals....... well i won't even go there, its a disgrace!

Here's a pic of my bike http://www.bikepics.com/pictures/1179610/

Cheers

Like i said before, the papers would be good for china but not too sure about in your home country. 

The only bike i've ever known anyone to bring back to their home country was one of the old Chang Jiang 750 military bikes.  But thats a legal chinese motorcycle.

Cheers