Thats what I needed to know! One more question: The screen capture on your first post, was this taken while using PlayUO? If so, download Razor from the following link:
Razor
Edit: OK, I can confirm now that the problem you have is related to PlayUO. I never used that before, but i installed it and tested the ToT trade window and have the same problems as you have. Please install Razor and when first started, put in the path to your "Ultima Online" client (something like: "c:\Program Files\EA Games\Ultima Online Mondain's Legacy") and check the box labeled 'Patch client encryption'. As Server put in '127.0.0.1' (or your local IP number as 192.168.xxx.xxx) and the port 2593 (if that is the port you use). Hit the OK button and it should start your "Ultima Online" client.