A fundamental assumption underlying much of mechanism design is that buyers know their values for the products they are interested in. We consider settings where agents receive signals related to their values from a joint distribution, and their estimated values are functions of their own as well as others' signals. We consider revenue maximization in such settings and show that a variant of the VCG mechanism with admission control gives a constant approximation to the optimal expected revenue. Our results do not require any assumptions on the signal distributions, however, they require the value functions to satisfy a standard single-crossing property and a concavity-type condition.