Unless he doesn't reveal it until the last Mountebank. Then you wouldn't know he has one.
Then he takes on 2 Coppers and 2 Curses for nothing... Or if he was discarding curses, that still doesn't make sense because he couldn't know you had THREE MBs, and he wants those Silvers. To not reveal Trader the first time would be very suboptimal (especially throwing in that it meant you would have $6 to spend instead of just $2).
Oooh! I got this one. Your opponent has a lot of Gardens, Fairgrounds, and Feodums, 10n+6 total cards in his deck, and 5m+4 distinct cards in his deck, for some m and n. He had previously trashed all his coppers for some reason. He also has an exact multiple of 3 silvers in his deck, so gaining two more doesn't give him any more points. Two piles are empty (probably the Gardens and Fairgrounds) there are exactly two silvers left in the supply, and there is more than one province. You are ahead on points and there is no +buy in the kingdom, so you can gain at most 6 points from a province buy this turn (your gardens, feodums, and fairgrounds couldn't power up no matter what you get), and can't end the game on piles unless your opponent reveals his trader. If he reveals his trader to the first mountebank, the game ends with his loss. So he doesn't, gains copper and a curse, but now has 5(m+1) distinct cards ... which puts your opponent ahead in points but not enough to win if you buy the province. When you play the second mountebank, that adds two cards to his deck but takes away a point. When you play the third mountebank, that would give 10(n+1) total cards in your opponent's deck and so he now takes them as silver to end the game ahead of what you could possibly get from provinces.