Coin values of any items is LCO.
The reason is two fold.
1. Maintaining a list of every chapters items, with the current system, would be a nightmare that I would wish upon only a select few people.
2. When you 'sell back' the rep for coin, you are in fact giving an item back to the game to be reused. That item already belongs to the game you got it from, and this is encouraging you to turn it back in.
More Item reps are lost due to people keeping them, or just never turning them back in. Which is fantastically disappointing, as we spend a fair amount on reps, and to have anything that is better than the $.25 ring that you get in the candy machine never come back is really quite frustrating. It does not encourage us to go get cool reps, as it does not make any sense to spend $10-30 on a rep that will never come back, when you are putting out 10-15 reps per game.
Personally, I would be a little pissed off if I found out people were donating our reps to another game, or 'selling' them there for their production value only to have that game use my rep. I have had people try to sell us reps that are not to our game, and I have told them I can't do that, for the same reasons listed above. I have taken the item number and description, and attempted to find the correct game for them, so I can tell them who they might need to talk to in game or out of game to deal with it. I have also mailed reps back to games if someone has an expired item, and says, I don't care about the coin, just take it.
As for the value of the magic on an item, there is no 'correct' ingame value. It is whatever you can get for it. There is a standard people use, and that is based on production value, but a traveling team that will use an item more, lets say at 12 events might be willing to pay more than the team that will only be able to use it for 6.
Also the coin value on an item is also an unknown factor, unless you have the appropriate skill to consult the OOG evaluate item list, and not every piece of jewelry is magical.