The Buyers love the location, are head over heals with the layout, the décor, the room sizes, the kitchen, the back yard, they're even talking about furniture placement...and then you walk down the basement and everyone is quiet...the damp, musty smell hits you like a ton of bricks! The Buyers run out so fast, you barely have enough time to turn off all the lights and lock-up!
At a time when Buyer are very, very picky about every last detail, this should be a major red flag to anyone 
Selling a home. I've shown too many homes where water has found its way into the basement and the real shame is that many of these conditions could have been avoided with basic preventive measures.
This can also hold true for crawl spaces; when was the last time you went into your crawl space? The next person who will most likely venture down there is the home inspector for the Buyer of the home. What will he/she find???
And we know all too well where there's water or moisture, termites and mold are sure to follow. The little critters love soft, moist wood to chew up. And there's a fungus among us, mold - seems grows on anything porous, out in the open where you can see it and behind the walls of that nice finished basement.
An ounce of prevention, I suggest the 10' Foot Rule:
- Clean the gutters and make sure there are no obstructions.
- Extend downspouts at lease 10' away from the foundation.
- Grade the perimeter soil (with clay type dirt) by at least 10'. Using mulch or soft dirt as a grade doesn't cut it and is a waste of money. You need good, hard soil to divert the water so it doesn't soak the ground.
Equally important is the reveal: the exterior soil line should be at least 6-8" below the foundation sill plate to insure water is not entering at the top of the foundation into the basement.
A sump-pump is a good idea in any basement where the water table is high or minor conditions exist that warrant it. Add a check valve so water doesn't re-enter the pit. But it becomes worthless if there is a bad storm and the electric goes out for a period of time. Install a battery back-up system!
If you do have a sump pump, make sure you check with your local government's requirements as many do not allow this water to be diverted to the public sewer system. And...follow the 10' rule, make sure the water is diverted away from the home.
A dehumidifier is a great way to keep your basement dry. If you do have a sump-pump, you may want to consider running a hose directly from the system to the sump so you'll never have to empty the container and run the risk of it filling and then not working.
Obviously if the home has cracks in the foundation, hydrostatic pressure from area groundwater, sewer back-up issues, the home was built on an active stream, has a high water table, etc., etc., and/or there are other mitigating factors causing water to enter the home, standard preventive measure are simply not enough and a professional waterproofing company should be consulted.
