It would not be zero, because you sold it, therefore it has value. Almost everything has some value. Inherited assets get a stepped basis to their fair market value at the time of death. Any reasonable valuation will usually be accepted as long as it can be explained. Depending on the amount of taxes you are going to pay on this sale would dictate how much effort you wish to put into calculating a basis. The 36 - to 48 month basis is a valid method and easy if you have access to 1099's or the decedent's schedule E. You can use the Estate Tax Return but the goal on the Estate Return is to get the lowest reasonable valuation for the estate. Your goal is the exact opposite.
If none of these easier calculations are available to you, you could make a case for taking your sale amount and reducing it by the annual inflation rate by year for the last 6 years. The key is whether it is a reasonable method of valuation. If it's a significant amount of taxes, talk to an accountant or attorney that specializes in estate planning.