Are you referring to Gothic 3? If you are, the only way to sell a certain number of items is to double-click an item, which will drag the whole number of that item into the sell box. Then, keep right-clicking to remove the item one by one until you reach the number of items you want to sell. That's the only method I know of.
__________________ "I have seen the blood and dirt on their faces. I’ve seen young boys turned into soldiers. I’ve seen men ripped apart by bullets. I can’t forget these things I have seen. And so I ask myself: How much more can one man take?" -Sgt. Matt Baker
you can also right click to move items 1 by 1 to you selling list, it works both ways.
Right clicking also works? Hmm. Maybe I should try this out. I really wish that there is a slider to adjust the number of that item you want to sell. It's not annoying, I agree, but it can be a bit tedious at times.
__________________ "I have seen the blood and dirt on their faces. I’ve seen young boys turned into soldiers. I’ve seen men ripped apart by bullets. I can’t forget these things I have seen. And so I ask myself: How much more can one man take?" -Sgt. Matt Baker
If you are, the only way to sell a certain number of items is to double-click an item, which will drag the whole number of that item into the sell box. Then, keep right-clicking to remove the item one by one until you reach the number of items you want to sell. That's the only method I know of. 
Linear Mode
