Waka is a community resource that tracks the approximate value of NC items, based on real-world trades.
It's a super helpful starting point, but it's not the only factor! Rarity and popularity can affect trade values too, and the trade economy can change quickly.
Be sure to take all factors into account for your trades, rather than sticking to just this number! And consider asking for a "value check" on the Neoboards if you're not sure.