Posting "rare" goods trade is more beneficial for the whole community than taking someone's hat trade and leaving own merchants idle, as it lowers amount of "rare" goods on the market even more. Half of those returned merchants would anyway return before their owner logs in and be able to send them back, doesn't matter if their trades were taken or not.
It matters to me that my merchant is returned or not. It doesn't matter to me if people post trades or not. More posted trades for rare goods does not help.
You seem to think that more posted trades for a rare good is a good thing. It isn't. Those trade goods go to the small group of hoarders who obsessively scan the listings looking for those trades. Accepting trades allows for a fair distribution. Maybe for half the people it doesn't matter, but for the other half (like me), it does matter a lot.