...If you post a note for sale for a very large amount from the ROTH account, and then purchase it from your cash account,
As I understand it, the IRS does consider all your accounts, including nontaxable accounts, when interpreting the wash sale rule.
Besides the wash sale issue which others have mentioned, when selling to an account or entity you control (your IRA or a corporation you own, or perhaps a relative or other conspirator) at a price at which others in the market would not transact, that's not a market price, or said another way that's not an "arms length transaction". Faking up a price this way to reduce your tax also violates something, even without the wash sale issue, but I don't have enough energy to go find the appropriate paragraph in the IRS code to quote for you.
So in other words, if instead you buying from your own IRA, and triggering the wash sale rule, you bought from Joe's IRA at an inflated price (and in return he bought from yours), there would be no wash sale rule issue, but there would be a violation of the rule about real market prices, and the IRS could come after both of you.
I've heard much more complex schemes, some pitched to me by accountants. One pitch many years ago involved chartering a bank in some little island country, and then a bunch of transactions between this bank and a corporation they would create for me, which went around in a circle in a way that was supposedly ok in the end because of a typo in some paragraph of the IRS code. (Sorry this sounds vague. It was all a white-board talk, as they refused to write any of this down on paper.) Fake is fake. I threw them out of my office. Couple of years later I read a Wall St Journal article about these guys. Govt had shut down their scheme, and IRS was going after big bucks from all the customers who fell for this nonsense.
FYI: IANAL.
