I'd like to remove items from merchaant I've made. I sold him over 9000 items and now the barter window is loading a lot of time.
No container.

How can I remove all his items?

(I don't want to discuss that is this stupid or not)

No script required. Just open up the console, select him, and use RemoveAllItems. Then give him a few days to respawn his normal inventory.

... hmm.... you might also want to check his normal inventory in the CS and find the formID for his key, and give it back to him with AddItem 00xxxxxx where xxxxxx is the formID of the key.