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.
what's a basement?
i don't think we have foundations either.
now if you want to know about water, moisture, termites and other critters we're experts.
Wet basements and mold = buying opportunities! Don't walk away from that a house that needs some TLC -- get the buyers to write an offer on this house that they otherwise love and just tell the sellers, "The reason the offering price is $10,000 lowerer than your asking price is that we're going to have to do the work to fix the water issues that you have on your list of things to do but just have not gotten around to fixing".
Of course, if the Buyers refuse to do any work, or to hire others to do so, then they're going to pay full retail for a house that is properly prepped. I do try to get my Sellers to do all of this work up front, so that they can achieve a sale at the maximum price that the market will bare. Yet in reality, not every Seller is going to be physically able to do the work. In fact, quite a number of Sellers don't even believe this is a problem -- they accept water in the basement and mold in the shower because they haven't seen many homes without these "features".
IMO, wise Buyers think more like investors, "I want to find a house that has some major issue which I can use to rachet down the price in order to make that quick fix, then gain instant equity". I think we might be raising a generation of whimps when it comes to home maintenance. Another 20 years of this and home owners won't know how to strip woodwork or wallpaper, sand floors, repair minor plumbing leaks, rewire an outlet for GFCI, Wait a minute. What do I mean another 20 years! That generation appears to be already upon us already!
Look at all of the young people moving into ticky-tacky townhomes and condos that will need all the windows replaced in the next 10-15 years, a new roof, maybe new concrete in the driveway. And then they live there for 3 years, put a sign in the ground and want to move to a real house, with a wet basement! (I'm not talking about the quality townhomes, I'm talking about the ticky-tacky ones).
There is an awful lot of cheap stuff on the market that looks pretty and smells clean but will be junk in 15 years. I'm sorry John, but I think the buyer's missed out on a great opportunity to do to the house what you recommend. Perhaps the thing to do here is get your Buyers back to that house and get them to make an offer for what the house is actually worth before they walk away. They just might be able to add $10,000 in value for only $2,500 in sweat equity. And, in a Buyers market, you just might be able to get that doggy offer to hunt!
Hi Jay: Sure, depends on what part of the country the home is in, realizing that not everyone has a basement. But the basic tenants of mold and termites apply to any home.
Hi Andrew:
Granted. Never say never. I agree. My concern in writting this post is that I've been on soooo many home tours and home inspections with the same recurring comments out the inspector's mouth: water diversion, water diversion. if only there had been water diversion.
Hi Lenn: My all time favorite is a home that I once showed where the sump-pump PVC-out pipe was broken and sat 2" from the outside wall where water was re-entering the basement, which caused all kind of issues. A $20.00 fix.