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Global chat would still be part of the filters. Also, there might not even be a global chat (and I sure as hell hope there isn't). And as the right screen shot in my post shows, you can also just filter out particular words (i.e. wts, wtb, wtt, etc), so even if there is a global chat - you can still tune out any and all potential spamming.
And as the final defense against such spam, GMs could enforce the "trade posts only for trade chat" rule, if there is a global chat after all. Or maybe there could be some simple chat bot that redirects any trade messages from global to trade.
In other words, there's definitely ways of avoiding trade spam. And as others said, there might be some other issues in your suggestion. Also, afaik Intrepid employed an economist to build out their economy system and trading might be included in that, so if the current design remains - it'd mean that a professional economist said "yeah, that's good design".