but IRS will sue you if you try to deduct losses that way
Guru is correct that the mark-to-market election cannot be made retroactively. There is an exception for a "new taxpayer," which means a newly formed business entity that is filing its very first tax return. But in general, the election must be made in advance, and it becomes effective for the following tax year.
With that being said...
The IRS does not sue taxpayers. They don't have to. That's not how the system works.
The IRS has the authority to audit your tax return, make corrections to anything they think is wrong, and simply send you a bill. They don't have to go to court.
If you disagree with the changes and the bill they send you, there is an appeals process within the IRS. If you go through that appeals process, and you don't like the results, then
you have the right to sue the IRS, and you can try to convince the court that the IRS is wrong. But if you do nothing, the changes made by the IRS, and the tax bill, will stand. And after a certain period of time, you will lose your right to contest the amount you owe. Once that happens, the IRS can use their collection tools, such as liens, levies and garnishments,
all without ever having to go to court.
When the IRS conducts an audit of a tax return,
it is not a criminal proceeding.
The IRS does not have to prove anything beyond a reasonable doubt. You have the right to be represented by an attorney or an accountant, but the government will not pay for such representation. You don't get a public defender, even if you are penniless. You cannot invoke the fifth amendment right against self-incrimination. And the burden of proof falls
on the taxpayer--not on the IRS--to show that they are entitled to any deductions.
BMK