If the previous owners are not traceable then it is not your problem, it is the problem of the current seller to locate them and get the registered sale deeds executed on their names and on the names of their subsequent buyers.
You cannot buy a property from a person who do not own the property.
It would be illegal and invalid on the part of the seller to sell the property in this fashion and buying such property will become a permanent headache to you.
In fact buying a property without a registered sale deed or having a defective title to the seller, cannot make you an absolute owner with clear and marketable title to the property.
Hence you were advised to not to buy this property which is disputable or having no or defective title to the seller.
The cooperative housing society had already transferred the property to the allottee who was the first owner. He subsequently failed to get the property registered on his name to avoid stamp duty.
The subsequent buyer also did not bother about the registered deed on his name in order to avoid payment of stamp duty, this followed till the current owner, however at one stage this illegal practice will come to an end.
Therefore you are advised to not to buy the property till the title of all the sellers are confirmed from the beginning till date.