A seller of an item faces a potential buyer whose valuation of the item depends on two private signals. It is well known that when there are informational externalities and the buyer's private signals arrive all at once, it is impossible to implement an efficient sale. I show that if the buyer's...

For mechanism design with independent values, we identify a subclass of Vickrey--Clarke--Groves (VCG) mechanisms that induce efficient ex ante investments even with externalities. The Vickrey second price auction does not belong to this class.