let me attempt to clarify my suggestion.
Presume I currently have 71 CHA. and I loot a Diamond. Now loot will only sell to PC's for what they are willing to pay, depending on rarity, need, etc, so the valuation skill would have to be based on NPC Vendor prices, which is fairly stable depending on very few variables. Let's say a Diamond is worth 150PP in a city where I have Dubios faction (I think this is the lowest faction you can have and still sell, maybe it is indifferent), and it sells for, say, 190PP where I have ally faction. (I read somewhere that faction affects prices of items). Now if I did a valuation on the diamond it would say something along the lines of " you think a Diamond is worth between 150 and 190 Platinum Pieces". Alternatively it could say "you think a Diamond is worth approximately 170 Platinum Pieces" (The Average of the lowest and highest prices you could sell for. This may appear to be a lot of work, but presumably the formulas for working out what loot is worth are already in the game for when we actually use Vendors to sell an item.
There could also be different messages depending on success/skill level. i.e. "You think a Diamond is worth 10 Platinum Pieces" or "You think Leather Arms are worth 200 Platinum Pieces" and also "You have no idea what a Diamond is worth".
Alternatively, there is another break off of this skill that would fall into Rogue style skills and that would be an "availability"style Valuation. Let's say I loot the above Diamond and use my availabilty (probably not the right word to use, but I cannot think of another at the moment) skill. The message would read (for a success) something like "a Diamond is a very rare item" or in the case of something that is not common but not overly rare "a Diamond is an uncommon item" or for a common item "a diamond is a common item". This would give new rogues the ability to judge roughly what an item would be worth to Player Characters, as a rare item is going to be of more value to a Player than a common item.