The share certificate is not a title document to confirm the ownership of the proeprty to the owner.
The share certificate is just recognise the owner as a member of the RWA.
There is no such bylaw forbidding the new owner to become a member with an application for membership without the signature of the seller.
The sellers might have sold the property and absconded to avoid payment of transfer fee to the society.
The registrar has not insisted on NOC from the society to register the sale deed in favor of the buyer to purchase the property.
All these indicate that these are not mandatory requirement as per law, hence the society can use its discretion as an conceptional case to issue the transfer certificate in favor of the buyer without the signature of the seller.
If the previous original share certificate has been reported to have been lost, then the formalities like a police complaint, a public notice in a local newspaper and an indemnity bond from the member may be obtained to protect the future the interests of the society.
It is discretionary and not mandatory, hence you can take a decision after a meeting with the MC members about this in case you feel that your individual decision may put you in trouble at a later stage.