  We consider the problem of a principal who needs to elicit the true worth of
an object she owns from an agent who has a unique ability to compute this
information. The correctness of the information cannot be verified by the
principal, so it is important to incentivize the agent to report truthfully.
Previous works coped with this unverifiability by employing two or more
information agents and awarding them according to the correlation between their
reports. In this paper we show that even with only one information agent
truthful information can be elicited, as long as the object is valuable for the
agent too. In particular the paper introduces a mechanism that, under mild
realistic assumptions, is proved to elicit the information truthfully, even
when computing the information is costly for the agent. Moreover, using this
mechanism, the principal obtains the truthful information incurring an
arbitrarily small expense beyond whatever unavoidable costs the setting
dictates.
