Artwork, for example, normally does not receive a specific value until someone purchases it at an auction. Other unique items that can be equally difficult to value include antiques, some collectibles and vacation homes.
If you want to know the value of these assets for estate planning purposes, you will need to have them appraised by an expert. After you pass away and they become part of your estate, they will likely need to be appraised again for tax purposes. You can go without appraising them for planning purposes, if you want. However, it will make planning more difficult.
Another thing to consider is whether you want unique objects to pass to your heirs through a will. This will likely make their values publicly known through the probate process. That public knowledge can be avoided by using a trust instead.
An estate planning attorney can advise you on creating an estate plan that fits your unique circumstances and may include the use of a trust.
Reference: Cleveland Jewish News (March 15, 2018) "Managing unique assets means obtaining appraisal."