Mmm.. in the broker window when you click on an item for sale, on the bottom is shows how much they put in as their price before 10% broker fee or whatever. So, just make your price lower than theirs.
Also, let's say the cheapest thing on the broker costs 45 plat and you happen to have a 10% broker fee (veteran display case). You want to make your broker price be 44 plat so how much should you put in? Take 44 plat and divide by 1.1 which give 40 plat. If your broker fee is 20%, divide by 1.2.
In the default UI, you can double-click on an item in the Sell tab and it will look it up in the Buy tab.
The DrumsUI broker window is much more convenient and it has the items in the cases on the right side and the broker items on the left.