<prompt> Human: Hi PF,As the title says, I have a problem. I have been investing in and trading cryptocurrency for 5 years. I lack the ability to substantiate any claim as to the source of these funds because I did not keep or cannot access 4 out of 5 years of records of purchases and trades that led to this point. In my minor defense, there was zero guidance about taxes and cryptocurrency when I bought the initial coins.Here is where I screwed up:1- I bought many bitcoin on mt gox, bitinstant, and coin.mx. All three are defunct, I have no record of these purchases. Not in my email, not on my bank statements cash, moneygram, western union were primay payment methods. Purchase dates, unknown. I do know the prices of the cheapest and most expensive coin of this group as well as how many so I know the average price, but again, I cannot prove any of it.2- Many trades were made on various exchanges over the years, each being a taxable event. Of 5 years of trading, I only have records for the most recent years records. One exchange is defunct, one does not have user accounts and provides no way to lookup order history, and the other I have lost the 2FA access to. This is thousands of trades that I have not properly documented with unknown dates, prices, gains, etc.3- I bought another crypto, Ethereum, in early 2016. I did this by exchanging bitcoin for it. I have no record of this taking place. I sent bitcoin to a service that sent me the ethereum in return. I know the price of the ethereum and quantity but again, there is no documentation of this purchase.I want to cash out, pay my taxes, and buy a house. I just dont even know where to begin. Can a tax professional help someone who cant provide any documentation of such a thing? Certainly there must be some way to resolve this properly?The little price and date information that I do know exists only in my head. I dont think that quite cuts it with the IRS when they ask for proof. Im a bit worried, I dont want any problems with the government.-A Throwaway  <response>: Reasonable estimates are often accepted for cost basis. If you know you bought 10 bitcoins on XYZ, then you should be able to look up the price of bitcoins around that day and use that as your cost basis.If you dont have access to your old wallets and therefore have no information, then they are going to be pretty vague estimates. You probably would want to consult with a professional to get an idea of how to work that. 