Buying resale flat in Kanpur

I am in process of buying a resale flat. This flat is not registered and the seller is an investor in this case. The registry of the said flat will be done between builder and me. There is a broker in between. Now my query: 1. The seller wants agreement to sell. If the flat is not registered in his name, how can he do agreement to sell. 2. As per broker, after agreement to sell, seller will write a letter to builder to transfer his rights to buyer (me). Builder will acknowledge the same and thereafter give NOC, after which bank will release payment to seller and registry will be done in my name with Builder as 2nd party. Is this legally allowed. What precautions / documents to be taken care of. I will see all payment receipts, allotment letter etc and any pending dues from builder, who will give NOC anyways. Other than that kindly advice what precautions are to be taken. Also from what broker says only some private banks allow this kind of loan where resale flat is not registered and endorsement is taken.