Could just make it that exotics could only be traded with exotics, legendaries with legendaries etc, shards with energy, something like that. Then nobody is getting extra exotics/legendaries they are just swapping around those that are already in the game. To stop the spam/any chat you either open up an in game shop with the items you want to swap and someone comes along, makes an offer and you can accept or decline, or you have a preset message like when someone finds a legendary engram/shards and the little white text appears in the bottom left corner saying something like "Krazykid would like to trade 'name of item' for a 'name of item' so that everyone knows exactly what is wanted and people can't spam chat with any rubbish. + limit it to one post a minute or something. Would be a very basic trade but effective.