online broker base in tax haven

hi,

Are there advantages to use online broker from tax haven instead of american base company?

Yes, you get a free audit from the IRS.

Not that I've found. But using an "offshore" broker does have the big disadvantage of causing you extra paperwork and headaches trying to keep everything properly reported to the IRS. Between the FATCA Act and a few other international treaties, Uncle Sam will find you, and he will take all your money (and possibly your freedom) if he thinks you're trying to hide something from him.