Printable View
Although I know most people play on a fairly large screen, when playing on a smaller resolution monitor (1024x768)* the trade window doesn't centre along the Y-axis, preventing trades from being made. There is plenty of empty space above the window, it just doesn't use it.
*Windows XP with Firefox with normal toolbars and menus
Few tricks you can try is pressing F11 to go into fullscreen mode (F11 again to exit!), also try hiding your friends menu below, this should give you some extra space for the trade screen, if this still doesn't help you can try playing with the zoom on Firefox (rather than the zoom in game).
Hopefully this helps, if not let us know!
I've played it at that resolution, and had no problem at all.
Only time it's been as bad as you describe is when I tried it on a 7" tablet screen. Even the 11" netbook screen displays correctly.