It's okay... its good to be cautious. But believe me... nothing but authentic stuff has come from Let-Trade. And if you don't believe me... you can ask just about anyone. They'll tell you!
You are right though... any website claiming to sell "new" LV is definately not legit unless its elux. Let-Trade is a seller of used LV, which I believe is some sort of consignment type business. I believe at least one of the people working for them was previously employed by LV (but no longer work for them) ... that's why they are such "experts" when it comes to LV. They would never accept a fake bag to sell.
Oh... and I have seen 2 "new" LV bags on Let-Trade's site before. I assume they came from someone who bought them from a boutique and later changed their mind and could not return the item. They were not sold at a discount though.