1. DD is of Andhra Pradesh Gramin Vikas Bank. Bank handed over the DD to my buyer. Is this a safe bank?
This is a government bank coming under the control of the nationalised bank 'Andhra Bank', what is your doubt about it?
Why do you go for bank's trust worthiness, you are concerned with the sale consideration amount alone and not with the bank,
2. What documents should I be verifying before the registration as a seller?
You do not have demand for any document from the buyer, it is you who have to submit all the relevant documents to the buyer.
3. Can I ask for a loan sanction letter copy from the buyer?
You are not concerned with the loan sanction letter or anything related to the buyer's personal affairs hence you cannot ask for any such thing, dont do this blunder mistake.
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4. If at all buyer wants to cancel the deal. Do I need to pay him the amount he paid earlier? In this case What are my legal options?
If you keep tormenting the buyer with all such irrelevant things, the buyer may opt out to by and may even cancel the purchase, in that case he may demand not only the entire advance money but also an interest for it from the date of payment of this advance amount to you.
You may have to return the same, because the legally valid document for having received his money is the sale agreement, by which he can sue you before court for recovery with interest, you canot deny the acceptance or receiving the advance amount.