Bike ownership transition is pending, who will be responsible if there is an accident in the transit

It is a matter of trust than the law. And, any accident  is not a matter of routine occurrence.

If the possession of the vehicle is delivered and the buyer gives the acknowledgement in writing,if the accident occurs then the possessor can be held liable for the consequences of the accident.

In my opinion until vehicle is not transferred ( Registration) till then vehicle should not be handoverd to buyer and seller would be responsible if any accident occurred meantime.

Even if transfer of ownership of the vehicle is under process and some accident occurs the buyer shall be liable as the vendee (buyer) has issued delivery note / certificate to the vendor (seller). 

When a motor vehicle is transferred the insurance is also simultaneouly transferred. The buyer should first obtain an insurance either transferred from the seller or take new insurance on his own before applying for transfer in the RTO records. If registration in the RTO records in buyer's name is pending, already there is an insurance in buyers name and the insurer will be responsible in the event of an accident. Whenever a vehicle is sold and bought the buyer gives a written undertaking to the seller that he(buyer) will be responsible for any accident thereafter. Notwithstanding all this there are judgments saying that the seller will have vicarious responsibility if the RTO transfer is still in the process. Nowadays used car dealers buy the cars and do not take any insurance themselves. As there is a long gap between buying and selling risk of accidents in the interim period are high. Transfer of insurance is more important than transfer in RTO records.

You can hand over the vehicle provided the buyer has taken an insurance in his name