Value depends on many factors: supply/demand, number of available coins and so on.
When selling goods just use current market price.
Well thats basically it... supply and demand. That's all. The rest is mental masturbation. Sure, rigging happens all the time, so do breaking news that can lead to crashes or mega rallies, but its al supply and demand at the end of the day, so OP, preev.com delivers an average price of all the markets, I would call that a real price.