http://www.kashrut.com/Passover/pdf/Pesach_Gram_Q_and_A_2011.pdf
Chazal obligated each person to search
for Chometz on the night before
Pesach. [If one leaves town before
that time, he is still obligated to
search for Chometz the night before he
leaves, although no blessing is
recited for that search.] In the
opinion of many Poskim, the search for
Chometz is obligatory whether or not
one owns his chometz by the time
Pesach arrives, since once the
Rabbinic ordinance was enacted, it
cannot be abrogated regardless of the
circumstances. Consequently, selling
the house to a non-Jew does not free
one from his personal obligation to
search for Chometz.
A solution to this problem is to set
aside one room in the house, even a
small one, and not sell it to the
non-Jew along with the rest of the
house. That room should be cleaned for
Pesach and thoroughly searched for
Chometz on the night before Pesach,
with the proper blessing recited for
the Bedikah. One who will have already
gone out of town by the night before
Pesach should follow the same
procedure on the night before he
leaves – but he may not recite a
blessing on the Bedikah.
