You may have noticed that sometimes when you sell some items to an NPC you get something unexpected. You may have encountered a 'vendor recipe'! Items with certain properties, or specific combinations of items, can be sold together to fulfil a recipe. Listed below are a few useful examples:
There are many more than these, some of which are complex and result in powerful unique items or valuable currency. Try experimenting with different combinations of items. You might just stumble upon a new recipe.