That only happens if you get a promotion that forbids something your character is wearing/holding. Normally it just shifts into your inventory, but if there isn't enough room then the shop keeper automatically buys it. So always make sure you sell surplus gear before getting that promotion or you may lose something you really wanted to keep cos there's no guarantee you will be able to buy it back. When it happened to me one time I was forced to go back to a saved game or lose a really good item.
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