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To make it short, every retired item (or even sometimes actual items that are still available) have a cap value. This value depends on the popularity, the demand and of course the rarity of an item. When trading Neocash, one cap is worth up to 200-250NC ($2-2.50). That means if you have one item worth 2 caps, you could, in the best of events, get up to $5 worth of items off the mall OR another item worth roughly as much as yours.
When people send me lists, I automatically put the value at $1/cap, except if they ask me not too. On clraik, most items go for that rate and sometimes even less (for bigger ticket items). For example, the Baltazar Bag of Bottled Faeries could trade on Neopets for around $400 worth of stuff as it is worth around 300 caps (people on the boards also do discounts for such huge ticket items). On clraik, this item goes for $100-150. Usually, it is not needed to discount for items worth less than 15-20 caps, especially when they're popular.
I hope this clears things up a bit, but if you have any other questions feel free to ask.