I wouldn't want to trade for something I can't get, but if I play with a friend then I don't care how they play as long as they respect my way of playing.
I've never understood the animal crossing purists who dislike those who time travel... since the game has no real end to it i see no reason why you shouldn't play around with the time and complete what you want to when you want to!
Of course I'll invite them and let them into my town, but I'd prefer not to receive an item that I shouldn't have available at that time. If it's a rare item I might see early on but probably won't, I'll have no problem with it as long as it was legitimately obtained. Fortunately, since the holidays are being patched in, I wouldn't have to worry about people bringing over holiday items early.