Since people using Coinbase would be buying bitcoins using their bank accounts, so how exactly would people be evading taxes here ? Wouldn't all that money earned that they didn't pay taxes on would have hit their bank accounts first ?
If you both bought and sold bitcoins through Coinbase and had realized gains (sold bitcoins for more than they cost), then the IRS would be able to identify that fairly easily. If taxes were not paid on those gains, they would would be able to go after you for the taxes due on those gains.