This may be a slightly off-topic question, but imagine if someone creates a unique product or service and sells it on a web site for bitcoin.  They only list the price in bitcoin, and there is no comparable product or service on the market.  If you purchase this item, how would you calculate your capital gains if there is no reference fiat price?  I don't think you can just use the current exchange rate as a basis.  What if this product were made available a year ago and the BTC price never adjusted to reflect the changing exchange rate?