About domain trading
The conditions of first-hand sales (aka new registrations) of domain can be found from different domain management institutes as references. Here we are talking about the re-selling of domains, i.e. domain transfer. Different institutes pose different regulations on domain transfers, some even completely prohibited such actions, so we must go through case-by-case.
As for .com and .net domains, first an agreement has to be reached between the original and new domain owner. The original owner then contact the domain management institute to change domain information, and finally hand the password over to the new owner.
As for domain transfers concerning .hk , a form issued by HKDNR must be completed with signs of both original and new domain owners. The concerned domain must not be in a dispute case, and there should be no objection from a 3rd party. When the new owner requests HKDNR for domain transfer, a handling fee of HK$500 & the annual fee must be paid. The amount that the original owner has paid will become invalid once the domain is passed to the new owner.