It's really not that easy at all. Especially with art or jewelry. We can know the current value. It could even be a very famous piece of art.

But these types of things are found all the time in attics and basements. Art especially is moved around without sales records all the time, and jewelry even more-so.

Heck, I have things I bought myself that I have no idea what I paid for them.

But I'd sure be upset if I had to pay cap gains taxes on these things assume their prior value was zero.