You just take the current fair market value of the assets in question. You realize the gains on the asset you are selling, and you start your cost basis on the

You just take the current fair market value of the assets in question. You realize the gains on the asset you are selling, and you start your cost basis on the asset you are buying. Now thats easy for liquid crypto markets, its harder for when you are trading a paperclip into a house. At the end of the day, you self declare and you do your best to justify yourself. If the irs has a problem, they will let you know.