If by some chance Bungie did this (which they won't, and I don't feel they should), they would have to enact a huge trade-tax. It would have to cost glimmer, motes of light, strange coins, and a telemetry for the weapon type you're trading for--along with trading the weapon you don't want (of the same rarity of the weapon you are receiving in the trade). Plus, you would have to use a Trade Token to enter the Trade Lobby. How would you get a Trade Token? You would get one every time you preorder DLC.
The good news is they will patch in the Trade Lobby long before they do a fix for that Minotaur in the VoG, or the Carry Extra Ammo/lose ammo on death-glitch, or add vault space, or allow you to use ammo synths with a full inventory, or fix the net code in PvP...
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trade tax would be a great idea. say 30 motes from each player. that way you can't just get exotics super easy and you actually have to play the game to get motes of light instead of strange coins which are pretty easy to get and glimmer is super farm-able. motes from each player would also prevent trading for something useless while giving a friend something really good, because you'd actually have to pay a decent amount for the trade.