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Number of items in every shop is more-less constant. After you visit into shop randomly generated new items are added into shop and/or random items are removed from it. It is better, when in shop are less items, which You have sold into shop. You don't want to buy them back, so it's like wasting shop's room.
So, don't sell cheap items (normal boots and cloaks, empty belts and bags etc) into shop! Instead, sell them as junk! You will get the same price and a bit more room in shops for maybe more useful items.