I have found the technique that never fails on shop keeper i can post a video later on the no failure technique thats very quick

Step 1: go up to bombs (or where bombs would normally sit on the shelf)
Step 2: when shopkeeper turns up, immediately go down.
Step 3: just before you hit the door, press down+right diagonally and go through the door.
Step 4:

Easier way is grab item stand to the south of him by the door

Walk one step to the right as soon as shop keeper turns go left+ down out the door
Way easier and closer to door

This way is slower