I am looking to re-sell my property (and in parallel buy another in resale) - and maybe as a seller (and potential buyer), I can answer this question.
Ask for these bare minimum documents :
1. Most recent sale deed - photocopy will do.
2. 7/12 and Index 2 records
3. If it is in an apartment, ask for OC and CC (occupancy certificate and completion certificate).
4. Copy of most recent electricity and water bill.
5. Society formation details.
6. Society share certificate.
I got this information from a law firm I am consulting. These documents can be starting point. Beyond this, we need to look for a good lawyer for verification and documentation.