"
Regardless of the entire "not wanting to trade ____ person" angle, there's definitely one reason why GGG should absolutely make ignore lists apply to merchant trading: the trade is done in the seller's hideout.
Which means that it gives
literally everyone access to the seller's hideout.
Players who have had to ignore other players due to harassment etc will now (potentially) have no possible means of keeping those players out of their hideout, other than to avoid using the merchant system altogether. And there's quite a few avenues for griefing that a motivated bad actor can do, once they've gained access to your hideout.
GGG will do something when every streamer's hideout is full with players 24/7.