I don't think it's quite as straightforward as people are making it out to be here, especially when considering forks. If you bought BTC a year ago and sell BCH (or BTC) today, what was your purchase price?

There are currently a couple of dozen Bitcoin forks, each with arguably equal claim to be the true successor. So it's difficult enough if you bought before all the forks and are selling all of your BTC in one go now, but lord help you and your accountant if you've bought or sold any inbetween some of the forks.