If you're referring to PC, it isn't the trading that ruins the game's economy. It's the fact that they can be traded at all (drops/system). In short, trading guns kills the economy because players won't buy from stores anymore, because they can capitalize between each other for cheaper prices. If a guy wants a BAR, eventually with time, he's not going to go to the store to purchase a BAR. Instead, he'll look for a player that doesn't want the BAR who will sell it for a lower price. This causes guns to lose value quickly. The cycle will repeat, and sooner or later, inflation occurs since peoples' guns aren't sitting idle in their inventory, but now rather a happier owner who will put use to it.

Also, becauae you can trade unwanted guns for other guns, you won't have to shell/buy packs to get that new gun. When this happens, players won't buy any more packs which leads to a loss in revenue for developers. That is a fatal mistake for any game, seeing as Era PC has had multiple server resets in the past.

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If you were talking about iEra, the same reasons as above. Though, I don't see a problem in a trade system for items that are already droppable. Just no items that aren't droppable (i.e. guns).