Tamper Detection Tape
AirQES Tamper Tape w/ Void Indicator (60 yard roll) $34.95
If you should try to open the window, the tape will pull loose but leave the word "VOID" shining through the mylar. Cut to desired lenght; the backing peels off in one inch intervals.
Tell the seller that you're taping the lower level windows for his protection; that way he can prove to everyone that he maintained the necessary closed-house conditions. Reside is easily cleaned with an alcohol wipe.
