I've had success with truck mounted carpet cleaning using "Chem Dry" system. But I've heard many others complain about it. For me, they've done good work. If the stains come back after cleaning, so will the company. They'll come back and re-do the job. Also gave me some spot-cleaner for free. I'm not paid to say this, it's just my experience with a particular company/product/system.I'm not a real stickler. I get my rugs cleaned about once a year.

When sweeping, the grit is being flipped along surface of the floor to an area and then picked up in a dust pan. At work or school, if you give the floor a "white glove test" (wipe with a white rag) after a wet mopping the glove/rag will be dirty. In other words a quick slinging of a wet mop across the floor will not remove all of the dirt. If your home floor is cared for in the same manner, it will not pass the "white glove test". I love a clean floor, socks don't get dirty, bare feet do not feel gritty and dirty. Dirt on socks or feet will transfer the dirt to the chair cushions, etc. if you tuck your feet up in the chair or sofa. Think about how dirty chair/sofa cushions will become if there are children running around on the floor and then getting on the furniture. Their little legs are too short to reach the floor in order to sit like adults.

Were previous owners smokers? We redid a rental house & got new screens, 2 coats paint on each room, completely redid the kitchen, new cabinets, countertops, new appliances, light fixtures, etc. New floors & new carpet throughout. As we worked of course it was fall & had windows open. Right after realtor started showing the house to renters it got cold & we had to turn on heat, wow, did the house stink, we opened up that furnace door & it was dark dingy yellowed smoke, stale stink. so hours more of bleach water & cleaners before the place was pretty good. We added bowls of vinegar at night & hid them in the morning. by the time renters moved in it was pretty good. So try vinegar bowls where kids & critters can't get at them Open box of soda in closed in areas might help. Take a vent cover off & wipe out area of vent & rinse rag out in clean bowl of water, is it all brownish? Then it's probably the vents, if it is just dirty water with no smell that might not be it. I took a long skinny broom to couple of my vents a few times, got almost nothing out but I had gotten a high quality duck tape for attics & seams & so I guess nothing get in them to speak of. I have allergies big time, also clean out the area under furnace where air goes into to it from hall. I put a heavy piece of carpet pad that is covered with plastic on the floor under there & then wipe out couple of times a year by taking vent cover off. Of course no 1 smokes in my house but that area does get some dust. Yes fridge, freezers & humidifiers have drip pans that need cleaning or they can make you sick. Also if you have any drains that don't have water in the elbow sewer gas can come up & make you very sick, is there drain in center of basement floor- be sure to put gallon of water down it often or keep it covered, upstairs bathroom not used, get water in all the drains including tub or shower. Good Luck, Hope you find the problem soon.

Good point Bruce regarding the anticipation of buyers asking for an allowance to do things. I will bring this up to DH. Most of the upstairs has been detail cleaned. I have a few more little things to finish. As I re-read the posts above, shellac keeps coming up. I need to paint the side of the cabinets next to the stove where the years of crud had built up. It's cleaned, but I think the sealer will help. I don't know about the rest of the lower cupboards. I hope that the lower drawers and cabinets smell better after I used the Pinesol on them. I'll be going over today. The drawers look like they have always been covered. But still that old wood smell is present. I might mix up a bleach solution today and wipe things down if they still smell. The uppers are painted inside, but someone years ago toweled on some sort of adhesive and glued down thick card stock type shelf liner. Who glues down shelf liner. Its curling at the edges, and I've made a mess of it trying to get it out. I'm going to buy some of the nice shelf liner for the uppers and cover it up. I have fresh coffee grounds to place in the cupboards - someone suggested that for odor control. The outside of the house has been washed. The small flower beds have been stripped and mulched. Looks tidy. Termite guy has finished the job. Do people leave the refrigerator running while the house is being shown. Its an older small fridge but its been well cleaned, thought still has an odor inside. I could use bleach water to wipe down that too. Dishwasher has been sanitized with a fair amount of bleach this last cycle. I used a bleach containing Cascade liquid too. But I did pour in about a cup of chlorine bleach in the pre-wash and ran it on "pots and pans". The rinse water was spotless. I made an error on calling the siding steel. Its aluminum. Needs painting very badly, but I believe its in good shape otherwise.