Moderators have a responibility of informing users of aspects of their community. This can be done via warnings in trade consoles, or public announcements, or other general broadcasts. They do not have a responsibility of interfering with individual transactions. In fact, doing so may lead to problems later on. When the community grows to large for moderators to intervene in each transaction, what happens then? Do the moderators become responsible for a user's lack of knowledge when the inevitable bad trade does go through? Moderators can make it easier to find information about potential trading partners or the trade value of items.
There are situations where users legitimately have one sided trades - ie. a user is leaving the game and trading everything to another player, or a user wins a community raffle and receives an "expensive" item, or the user is new and someone is feeling generous. All of these are legitimate, and involving a moderator in these removes them from other trades where a real scam could be occurring. It also removes a moderator from other aspects of the community they could be moderating.
Another aspect to consider is whether it is the moderators' or even the developers' responsibility to determine the price of a shiny red pixel in regards to a sparkly blue one. Or is the market value of red vs blue determined by how much someone in the community is willing to pay? If a player wants to sell an item for twice it's "normal" value and someone needs this item right now, they are willing to pay a premium in exchange for receiving the item immediately. Over time, game items become more or less common and the price fluctuates. If it is the moderator's job to determine those prices and how every item compares in value to every other item, that sounds less like moderation to me and more like micromanaging.
Remember, it is a game. In a game, there are generally winners and losers. Whether you like how that sounds or not, that aspect of the game extends to trading aspects of a game too. Someone who is more skilled in negotiation and knows the values of the items is going to get a better price. An experienced player is probably better at it than a new player, just like the first time a new player steps into a server and the experienced player snipes him from across the map. A moderator doesn't step in on those occasions (hopefully), doing so just because the player is new in the trading area should not be a reason to interfere.