  When a company undergoes a merger or transfers its ownership, the existing
governing body has an opinion on which buyer should take over as the new owner.
Similar situations occur while assigning the host of big sports tournaments,
like the World Cup or the Olympics. In all these settings, the values of the
external bidders are as important as the opinions of the internal experts.
Motivated by such scenarios, we consider a social welfare maximizing approach
to design and analyze truthful mechanisms in {\em hybrid social choice}
settings, where payments can be imposed to the bidders, but not to the experts.
Since this problem is a combination of mechanism design with and without
monetary transfers, classical solutions like VCG cannot be applied, making this
a novel mechanism design problem. We consider the simple but fundamental
scenario with one expert and two bidders, and provide tight approximation
guarantees of the optimal social welfare. We distinguish between mechanisms
that use ordinal and cardinal information, as well as between mechanisms that
base their decisions on one of the two sides (either the bidders or the expert)
or both. Our analysis shows that the cardinal setting is quite rich and admits
several non-trivial randomized truthful mechanisms, and also allows for
closer-to-optimal welfare guarantees.
