In order to buy, sell... the quantity prompt (how many...?) is set to max by default.

Usually the desired amount is 1. Any mod?

Ask in a Morrowind Code Patch thread. Hardly anyone else can help.

As for awhile, try to Shift-click on goods instead of just clicking. I remember it selects a single item when moving between containers, maybe it can help when bartering too.

IF you click while holding Shift, you will select all items in stack. By holding Ctrl you'll select one.

Thanks for your answers.