Slot in merchantlist doesn't get changed by the server at all. Once you assign it, only you can change it. I think the confusion comes because perhaps you are thinking slot is an id. While it could be viewed that way, its real purpose is to allow the world builder to determine what slot he or she wants a particular item to show up in in-game on the merchant's list. This would allow somebody to say, put the more expensive items at the top of the list if they so wished.
In-game items will fill in to cover gaps in the slots, so there are no gaps visually. However, the slot column will still remain the same. If we have items with slot 1, 2, 3, 5, 21. The items would use slots 1-5 in-game, but the slot values in the database would remain 1, 2, 3, 5, 21. If the server did change that column, we wouldn't have a problem with slots overlapping, but we would also lose control over item placement on merchants. We want to tell the server which slots to place items in, not the other way around.
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