I think it's to get rid of all the bots that plagued the market previously, and it seems to have been relatively effective at doing so (judging from the volume on commonly traded cards like the L4D cards.)
I think it's to minimize the damage from the thiefs that hack user's inventaries to stole their items and then sell them.
But, why is a simple email confirmation enough for trades, but not for selling items? Just because they can get money from it?
Please excuse my bad English.
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