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When to Cancel Buy Orders and Sales Listings
My general rule of thumb is to wait at least 24 hours to cancel buy orders (no penalty) and one week for Sales Listings (5% loss each time).
If you EVER accidentally list an item for gold instead of silver, sometimes it isn't worth it to even bother cancelling it. This is because the 5% penalty is based on whatever you listed the item for.
Now, why do I wait so long to cancel sales listings? Obviously there is a 5% fee, but the real reason comes down to a weekly cycle that the game naturally goes through.
Every Thursday prices across the board, prices tend to drop, particularly in the morning (my time is EST). Then Friday night the prices rise going on into Saturday. Since it takes a week for this cycle to complete, I usually cancel my listings after that time period is over. If I post on Friday and it doesn't sell by the end of next Friday, then there have been two great chances for the item to sell but it didn't. That means it is time to cancel.
I rarely, ever, have to cancel sales listings. Buy orders however, I cancel about half because they don't get fulfilled. I usually over place the buy orders since I'd rather some didn't get fulfilled than miss out on opportunities.
Hope that helps!