The the law is clear in that the inherited value of the house is the FMV on the date of death. Period. Now it used to be that proof of value would be an appraisal that was two or less years prior to or after death. I've not seen anything "official that changed that. Though I have "heard through the grapevine" (i.e.; rumour) that an acceptable appraisal needs to be (note "needs to be" not "has to be") within 6 months either side of the owner's death.
Now if you sold it through a real estate agent or agency, that agent or agency did whats called a "comparative analysis" of similar homes within the same area that were sold within the prior year in order to come up with a realistic "value" for selling that property. That comparative analysis, which you should have a copy of, is generally enough to substantiate your cost basis should you ever be audited on it.
Now most likely you sold it for less than your original asking price. So using that original asking price or the price you actually sold it for would be acceptable as your cost basis. Note that if you use the original asking price as your cost basis and sold it for less than that, then your "loss" would not be deductible. You just wouldn't have any taxable gain on the sale is all.
