Now you’ve got me wondering how it would happen if the system was up when traders changed. Could you be in the middle of a trade, and the new trader takes over and doesn’t have the item?
There was a time when trader flip consistently conflicted with server maintenance. Back then the trader flip happened while the server was offline and when the server came back up you could see the results of your bids.
Every time trader flip has conflicted with a maintenance this pattern has remained; the flip happens "on schedule" but nobody will know the outcome until the server is actually available.
There is the one exception where there was a week where trader flip simply didn't happen, but I can't even remember if there was a conflicting maintenance on that one occasion... IIRC, that was an issue caused by changing the time in which flips were supposed to occur and we essentially got two weeks for the price of one.