If the vehicle has not been legally transferred to the buyer's name then the original owner shall be held for any mishap including an accident claim or if the bike has been involved in any illegal activity.
Law will not understand your explanation that you did not have time to transfer the ownership legally to the purchaser even after receiving the money.
If you have the address of the first owner you can write to him stating that he never bothered to transfer the vehicle to his name after buying it and that yo ware willing to do it by following due process of law and instruct him to cooperate with you for this or else to return the vehicle and take back the money he paid.
You can endorse a copy of this communication to the concerned RTO.
If the buyer do not respond then you can intimate the fact to the RTO by enclosing the copy of this letter and request for making an endorsement in the register about the transfer of the vehicle to so and so person by the original owner on so and so date.
When the owner of the vehicle wants to dispose the vehicle i.e sell the vehicle or gift to someone or change ownership after death of owner etc, transfer of ownership can be carried out by RTA and STA depending upon the class of vehicle
Where the ownership of the vehicle transferred, the owner of the vehicle should intimate to the registering authority (RTO/MVI) and the owner will be held responsible for any legal issue if the vehicle not transferred /change of ownership.
Its is very important that seller or previous owner should transfer the ownership to new buyer at the time of selling the vehicle because seller or previous owner will be in trouble and legally responsible for all the offences carried out with the vehicle by buyer or new owner without transfer of ownership such as traffic violations, traffic offences, accidents if any, crime committed with vehicle etc .