The only reason the sword would disappear is that you may have completed the quest for the statues and the paladin has changed into the undead version of the Furious paladin. In that case the troops and the inventory change.
The same happens sometimes when you complete main story quests. Most of the time you get additional items to the one in store (greenwort is a good example) - but I had a monk's belt (the only item left in store) being replaced by a few new items.
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