Here are some tips on how to prepare a house to be vacant.
A vacant house is going to develop smells. You will open the door and it will smell musty. Every smell that was in that house is going to be amplified from having been closed up. Houses need to breath. Dirty carpet is a big culprit for odors so have the carpet cleaned. You could also use Febreze Antimicrobial; it goes a long way to making a place smell good. It kills the odors by destroying the bacteria that cause the odor. It does this with odor eating enzymes.
For pet odors, there's a product called Simple Solution that also has bacteria killing enzymes. http://www.simplesolution.com/products_us/so_remover.html I sprayed it on some outdoor cushions after my cat urinated on them in an effort to mark where a foreign cat has laid. Simple Solution really did the trick. I testify it works.
Make sure the air return filter is replaced with a new one. You don't want that dirty musty smell to keep being recirculated. A new filter will cut down on your dust too. Get a good filter; one that you can't see through. It may be one called Filtrete by 3M.
The toilets in a vacant house need to be tended to. First they should be cleaned thoroughly and then some scented bleach put in the bowl to sit. Keep the lid UP; a down lid will hold the moist air in and cause mold and mildew to grow under the seat lid. If you don't do bleach, flush the toilets every time you go check on the property or once every two weeks.
Know also that when you get ready to do the house inspection with your buyers there, you should go beforehand and run out the water in the water heater. It probably will have developed a strange odor from having sat for so long.
Spiders, they're everywhere. Spiders will build a home faster than you can say sold. Know that general pest sprays will not kill spiders. The can should say "kills spiders". Spray the corners of the rooms with it as a prevention.
For general bugs and roaches, I recommend using foggers. Treat the attic, interior spaces and under the house all at once for an effective treatment, otherwise the pests just migrate. Having a roach show up to your open house or while you're showing is not going to make a good impression.
It might also be a good idea to turn the water off at the meter to a vacant house just in case there's a leak that develops on your watch. I also recommend to anyone with a washing machine that they replace their rubber hoses with wire mesh hoses. They won't burst on you. They run about $12 a piece. #E9WM60 Washing Machine Hose | Hat Creek Outfit It's a great investment and peace of mind knowing that you won't come home to a flooded house...at least not because of a burst washer hose.
In the kitchen, put some dish detergent in the disposal, turn on the hot water and turn it on to give the disposal a good cleaning. You don't want foul smells wafting up from the disposal.
Lastly, get some scented plug-ins. They do wonders. Keep that vacant house smelling good. Remember, smells linger and so do impressions.
One important thing for your sellers to know is that when a house becomes vacant, some insurance companies will go up on their rates so it may pay to hire a house sitter or get a temporary renter.