here's my thinking. under the current law, if you sell at a loss in a taxable account and then violate the wash sale rule in a nontaxable account, it doesn't matter whether the a/c's are with the same broker or not. the wash sale rule will apply. So it's not the a/c or broker that matters. I would also conclude that if an H&W each had their own taxable accounts and one sold a security at a loss and the other purchased the identical security during the 61-day-period, the wash sale rule would apply. Thus, I would conclude it doesn't matter whether you have the securities (all taxable accounts) at one brokerage - multiple accounts or multiple brokers. However, if you ask me to point to something specific in the IRS rules, I can find nothing that's current. And of course with multiple brokers, each would use what they have as the tax basis to report the sale. Even with multiple accounts at the same broker (H&W accounts), when stocks were sold at a loss, I've seen the account use its cost basis with no attempt to apply the wash sale rule because substantially identical stock was purchased during the 61-day-period in the other account. Nor have I seen the FIFO basis in the other account used if those shares were bought before the shares that were sold in the first account. And to be honest I've never seen an attempt by professional tax preparers to do other than report what's on the 1099-B. Their problem is that they don't have full information to do otherwise.
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so perhaps to best thing is to contact the brokers, though I'm sure they'll tell you to talk to your accountant.
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I guess a question would be what do CPAs do when auditing the financial statements and preparing a tax return of a client that has multiple locations with identical goods (inventory) but bought at different times and costs? Would they treat each location separately in determining the FIFO cost of each location's inventory and then add each to determine the total FIFO cost or would they combine all purchases for all locations to determine the FIFO cost for all stores as if there was just one.
