I use to use C0inbase during the end of '2014' and a bit in '2015' as this was my favorite exchange up until I heard and joined 'Circle';) Matter of fact, I owe 'C0inbase' m0ney as I discovered a flaw in the early stages of up-in-coming bitcoin exchanges;)
First, didn't the IRS say themselves that they didn't or don't currently know how to tax bitcoin and that it would be considered "property tax"? Yes, if you were to convert to fiat, it's up to the user to take care of his/her business in dealing with putting money to the side for taxes. Hell, I owe taxes what the hell do I know, right!