Get your documents in order: Find your closing statement from when you bought and or refinanced the apartment. It may contain some useful documents. If you don’t have a mortgage, then find the stock and lease. If you have a mortgage, they should be with the lender, although sometimes lenders lose them and you have to go through a usually easy but sometimes lenghtly and painfull process to have the co-op generate a new stock and lease. It is a good idea to keep your last mortgage bill because it will have the contact information necessary to order a “loan payoff”.
If you are the executor of an estate, ask your lawyer what documents you will need to sell. Find the death certificate, make sure the estate taxes have been paid, the certificate appointing you as executor are the minimum things you will need.