What documents should I check before buying a resale flat?

At minimum, you should review the original sale deed (and, ideally, the chain of prior sale deeds going back several years), the encumbrance certificate to confirm there's no existing mortgage or other charge, the latest property tax receipts, and the occupancy certificate for the building.

Also check the society share certificate and any NOC needed from the housing society for transfer, confirm the seller's identity matches the ownership records, and verify there's no pending litigation or dispute over the property.

If the seller has an existing home loan on the property, you'll additionally need confirmation from their bank about the outstanding amount and the process for releasing the mortgage once you make payment. Given how much rides on getting this right, it's strongly advisable to have a property lawyer conduct a formal title verification before you finalise the purchase.