how to properly sell a bike

[bikerdoc]

The thing is that there are so many variables that there's generally no one specific answer to your question OP.
There are some areas in TIC where a second hand/used sales tax still has to be paid but not in other parts of the middle kingdo0m as an example. The tax is payable based on a % of the agreed sale price and has to be paid prior to the actual transfer at the Traffic PSB (second hand sales tax invoice required when completing said bike transfer). However, this may differ depending on local laws etc.

Generally speaking, the best bet would be that the buyer and seller both present to the Traffic PSB where the bike has been plated/registered and handle the transfer there. Great local language skills and a healthy dose of tolerance, perseverance and humour might be also needed on the day.
Expect to provide all the bike related documents for the transfer including appropriate PRC Drivers license for both buyer and seller, or at least have the same handy just in case. Passports also and the ubiquitous PSB residency slip of paper proving place of residence.
Some locations will require removal and forfeiture of the exisiting license plate on the motorcycle (under sellers name) and the buyer will be provided a new plate to be attached to the motorcycle. Some locations/jurisdictions require a bike inspection prior to the transfer from seller to new owner while others will require this as part of the transition from previous to new owner after some parts of the process are completed.
Insurances (CTPI) might be transferable from existing owner to new owner in some jurisdictions or not, as the case may be.
So generally it is easier and likely wiser to have the buyer and seller with the bike front up to the Traffic PSB where bike registration is held.
In the event that the buyer is from out of province then all the documentation related to the original registration etc will be provided in a large officially sealed envelope which the buyer would need to take to the relevant Traffic PSB in the their jurisdiction to the complete relevant bike registration in their locale. The envelope has to remain sealed and any evidence of tampering will void the contents at the receiving Traffic PSB. There are some special caveats to this e.g. Shanghai and Beijing have special local laws and regulations on vehicles first registered out of province.
I've sold several bikes and bought one out of province, and have found by experience that there are some variations as noted. Take every official document and receipt relevant to the bike vehicle and parties concerned, it's also worthwhile to have several photocopies of the same.
Good luck.