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I made only guide how to do it with 20slot bag.
1. You need 20 slot bag
Put it near your backpack
2. Put there items which you want to dupe
3. Apply filter
4. Shift-click on each item and take 19 from it and put it anywhere except last slot in 20 slot bag
5. Go to any vendor
6. Take any item from vendor and put in last slot of your 20slot bag(it should disappear)
7. Go to your guild bank and put those stacks there (you can trade it or send it somebody else)
You see there is 23 items in stack but its only client side, there are 19 in real.
8. Go to any vendor
9. Buy any item and put it in last slot of your 20slot bag again. It will disappear.
10. Relog
11. Go to your guild bank
12. Enjoy
If you are too lazy filter is
027
13
27
FILTER: