Bitcoin Forum

I'm trying to summarize my knowledge of Bitcoin tax rules in a series of examples, please correct my calculations if they seem wrong.
Calculating US federal taxes, assuming marginal tax rate of 25% and long-term capital rate of 15%.
Negative tax liability numbers mean you receive a refund from IRS.

1) Bought 1 BTC at Coinbase for $1000, sold 1BTC at Coinbase for $1200.
    Tax liability = ($1200-$1000) * 25% = $50

2) Bought 1 BTC at Coinbase for $1000, sold 1BTC at Coinbase for $200. Total capital losses for that year are less that $3000.
    Tax liability = ($200-$1000) * 25% = -$200 (refund)

3) Bought 1 BTC for $1000 from a stranger without receipt, sold 1BTC at Coinbase for $200.
    Tax liability = ($200-$0) * 25% = $50

4) Bought an ASIC miner for $500, mined 1 BTC during the year, and sold it for $1000, burning $100 worth of electricity in the process.
    Tax liability = ($1000-$500-$100) * 25% = $100

5) Bought 1 BTC at Coinbase for $1000, sent it to your own Mycellium wallet, exchange rate at date of the send is $1200.
    Tax liability = ($1200-$1000) * 25% = $50

6) Sold pizza worth $20 in 2010 for 10,000 bitcoins, sold bitcoins in 2015 for $200 each.
    Tax liability = (10000 * $200 - $20) * 15% = $299,997

7) Mined 1BTC in 2011, have email from Deepbit mining pool to prove it. Sold in 2015 for $200.
    IRS argues email is not a valid receipt, charges highest rate.
    Tax liability = ($200 - $0) * 25% = $50

Did you keep track of EVERY single transaction ever?
I think everyone has at least done some tradings in exchanges that are now closed and lost the trading history. This happened to me. I would like to know what to do in such situation, where you genuinely lost track of your BTC earnings/expenses)

Isn't it the value at date of taxation? Just like paying taxes, they check your bank account on the first of January? Then it should use the value at January 1, add it to your total capital and you pay taxes on the total value of your assets? Just like stocks?